Part One: It's Hard
Yeah. Merchants of Kaidan is hard. Excruciatingly hard. Don't worry, though. It's really only difficult in the beginning. If you've looked up this guide, you're probably having trouble so you're probably encountering these problems (which are the problems I encountered):
-Bad Profit Margins: The economic system the game forces you to operate in is really, really intense. In similar trade games (e.g. Port Royale, Patrician) you can buy and sell goods at the same location for the same price. In MoK, the system is slanted against you. Every good's selling price is less than its buying price at the same location by a given percentage. Practically, this means that if you buy food for 10 and go to a place where it is being sold for 11.5, you will not make a profit even though you are transporting a commodity from where it is plentiful to where it is more scarce. And you're right, that makes no sense and serves literally no purpose other than to make the game that much harder by cutting into your profit margins.
- Seemingly arbitrary economic system: So, villages produce food and cities consume it? But... you can buy food sometimes for 5 (which is about as low as you'll see it) at Orvoros and a neighboring village could have it for 12. Surely, Ore will be consistently cheaper in cities near mountains, right? Nope! Kaidan is an economic system where cities don't appear to have specialties, even though they may claim them, and prices change almost magically.
- Excruciating random events: Early in the game, the random tolls and attacks and what-have-you are minimal. The most excruciating thing that can happen regularly is that your carts break down and you're crippled, particularly if you just spent all your money on goods (which is reasonable) and you arrive someplace where it's cheaper than what you paid for it. When you advance in the game, you will be attacked by dragons incessantly.
These problems make MoK a very challenging trading game, but far from impossible. By far, the most difficult part of the game is in the beginning. Because you can only trade in four things, you're kind of screwed. You have fewer options and less of an ability to spread risk or take advantage of opportunities. For instance, if you have access to all the goods, your chances of spotting something that is significantly below average prices is much higher. Also, more expensive goods see more fluctuation and are frequently lower weight, meaning better profit margins on the stuff you do sell and selling more of it.
What's nice about the hardship is that is simulates real life trading insofar as you are really really concerned about maximizing your profit margins.
Part Two: Breaking Through
If you just want a few easy take-aways this is what you want:
Food: 8 trigons is a good price. You'll make a profit on it. 6 and 4 are spectacular. Never buy above 10. Food prices go up in winter, down in fall.
Hides: 12 trigons is about the bottom of the barrel. Below that is just gravy. 25 is about the most it will be sold to you for and 22 is the most you can usually hope to sell it for.
Olive Oil: Never buy it unless you have to for a quest. I'm serious. Just avoid it. I've never been able to nail down a pattern with its pricing.
Ore: 36 and under is a good price. Above that, I wouldn't bother given its high weight.
Hill-Herb: I think this must be a glitch. I never see anyplace selling it for less than ~130 or buying it for more than ~25. Not to mention all of the random patrols that will confiscate it from you.
Knowing these things will help you advance in the merchant's guild, giving you more goods to buy from and from there it's downhill. Really, you have to believe that breaking past the food/hides barrier is the single hardest part of the games. Your profit margins octuple after that because you can deal in expensive, light-weight things such as spices, jewelry, etc. Here is some info on some of the other commodities, but my knowledge of their prices is much less thorough.
Spices: ~65 is a good price to buy it at. It seems to be cheapest at Ovoros and most expensive at Urgune, making it a pretty good buy.
Clothes: Anything 21 and under is excellent. Clothes are great because their supply and demand remain relatively static. You can buy them for good prices in a lot of places, including some villages, and Ovoros seems to be a good place to sell. Demand goes way up in Winter, but not as much as supply. So clothes become significantly more profitable in Winter. Opposite is true in Summer.
Tools: 25 and under is an excellent price. There seems to be a lot of variety with tools so I play it safe with them, particularly because they weigh a good deal.
Jewelry: 75 and under seems pretty good.
Porcelain: About the same as Jewelry.
Part Three: Carts + Animals
Right. So a cart is a huge investment both in the short term and long term. They're incredibly expensive and they increase the amount you have to pay in taxes, tolls, and repairs (likelihood of wheel breaking goes up). So you should really only buy them at the right moment. The best moment to buy another cart is when you have a good deal to take advantage of (e.g. food at the price of 4 or something) and you can fill up with something you know is profitable.
Make sure you hunt a bit for the best bargain for earlier carts and upper level carts. Prices do fluctuate depending on the city. Doesn't seem to be much rhyme or reason to it, but I like buying in Urgune as their prices tend to be pretty good.
Now I want to break down the math of carts. So, carts will permanently increase your storage capacity. This means that when it would normally take you ten days and forty gold to transport 600kg from point A to point B, it will now only take you five days and thirty gold. And make no mistake: Your first cart purchased should be a level one cart, not a level two. Increase your profit margins immediately, then save and start upgrading your carts to level two, which provide even better margins (because your storage increases relative to the increase of your fixed and variable costs). So yes, buy carts. They pay for themselves very quickly.
As for animals, the jury is still out. I like them. Basically you sink 1000 trigons to permanently decrease your travel time by a certain percentage. Buying more carts means you have to buy more steeds to achieve that same percentage, to a maximum of 50%. The decrease in travel time not only ultimately saves you money and minimizes bad things that happen to you, but they also maximize your ability to take advantage of profitable events, advantageous prices, etc. It also seriously helps you on timed quests.
Part Four: Advisors
So, most advisors are completely useless unless you want to go delving into ruins and mines, which you are disincentivized from doing in the early game anyway. Because most have to do with either founding a monastery/mining operation after you clear one out or fighting whatever is inside. The two types that do not are craftsmen and traders.
Craftsmen are of varying use. There is one craftsmen/advisor that I will always, always recommend: The cartwright. The cartwright will immediately fix your carts that break free of charge in exchange for 5 trigons per day. So, in order to pay for himself you'll only need to have a break-down once every twenty one days or so which becomes an easily achievable reality as the number of carts you operate rises. Get this guy.
As for traders, I must give a firm "meh". Have I seen them be useful? Yeah. But their usefulness is not guaranteed and, when it happens, isn't overwhelming. Feel free to pick one up if you want, but I'd avoid the specialist (Empire/Barbarian) ones as they're next to completely useless for the vast majority of the time.
Part Five: Mines/Monasteries
Should you work towards establishing mines and monasteries? Yes. They are things that you can basically leave alone to chug along in the corner and make money. When you have level two and three carts, you're a Citizen in the Guild, and you've got about 50,000 trigons in assets and at least 15,000 of that in cash, that is when you should start worrying about them. Never before then. Because if you build up the manpower that you need to clear out the monsters, you'll bankrupt yourself.
Before you start hiring mercenaries like crazy, which you will do, you're going to want to get a Sergeant. That is priority one. Get used to checking the tavern in every town you go to to find a Sergeant and Monks/Foremen (you'll want at least one of each eventually so you have them on hand to found the monastery/mine). But the Sergeant is vital. Once you get a Sergeant, take him to Narheym and train him at the military academy so that he's level two. Then you're going to go back to trading and making money, the difference being that while you do it you'll be checking taverns for any mercenaries and hiring them on the spot.
If any mercenaries approach you on the road? Even better! You get them without paying the upfront cost. You're going to want to assemble a host of mercenaries until you have about 25 that are not managing carts. Then you can start clearing out ruins and mines. I, personally, prefer clearing out ruins to establish monasteries. Mines require you to supply them with wood, which can be expensive. Monasteries just take one of your monks and about six or seven mercenaries and chug away at no cost to you. Mines do provide a better margin provided you can get your wood cheap, but they're not maintenance free. Which is annoying.
Part Six: Miscellanea
Bribes: When do give them, when not to? I say that you almost never should. Obviously if they're looking for hill herb, don't bribe them because they won't find anything anyway. But if it's a road/war tax, I would still say you shouldn't give them squat. Because, until you reach a high asset amount, the bribe cost will be higher than the tax anyway! And, even after you reach a high asset amount, the chances of you getting booked for bribery are high enough to offset that. Just bite the bullet and pay the tax until the taxes get outrageously high, which, yeah, they eventually will. Then you should start giving out moderate bribes.
Old Lady: Pay. Her.
Robbers: I've never detected a difference between either of the options. By the time they'll do enough damage to matter, you should have enough mercs to kick their butts anyway.
Attacked Merchant: Don't pilfer. There's a good chance guards will catch you and you'll never find much anyway.
Chest: Open it.
Quests involving shady characters (such as Sly Joe): Take them and follow their instructions. The rewards are great and there seems to be no penalty. What's great about quests that make you go from A to B to deliver/pick up something is that you can trade along the way. What does it matter to you where you go so long as there are price differentials along the way?
Olive Oil: Never buy it. Yeah, it's on here twice.
Items: Always have them identified before you sell them. On average, you'll make much more money. And obviously, some of them you'll want to keep.
Sea/Air Travel: Avoid it. There's two reasons for this: You're charged a cost of travel on top of your normal costs and it's scaled to the number of carts you have, making your margins very very tiny. Also, you're missing out on potential money making opportunities along the route you would otherwise travel! So it has a real cost and an opportunity cost to you.
Saving: I save before every large/questionable purchase of goods. Given the huge lack of predictability and the pretty arbitrary economics, I do not feel bad about it at all.