In front of the Underworld there's a ghoul who is trying to sell Aqua Cura, which is just regular water but he's saying it will heal the people. You figure this out and if you threaten him, he will give you a cut from every weeks profit. (Broken Steel required) 
 
Then, if you also have Point Lookout, you can keep going back and forth on the boat to Point Lookout, which actually is a month-long trip in the game. This way, you get hundreds of caps every time you come back and forth and ask for your cut.  

Or you could just tape down a couple of buttons to gamble, like in the first Fallout.

"In Broken Steel" could of been added to the title to make this a little more relevant than how much it actually is. And surely if a man knows of the Amazing Aqua Cura and the other side quests in Broken Steel then I am sure they know about this

After the Ghoul selling Aqua Cura got killed, I could come back to his body nearly every day and he'd have 500-600 caps on him for whatever reason.

Just pick up every piece of ammo you find because it doesn't weigh anything. And then sell what you don't use. I never did any of the Aqua Cura stuff.

Just sell cigarette cartons to the merchant in the place where your house is, I got 16 to 20 thousand caps from doing that.

"Just sell cigarette cartons to the merchant in the place where your house is, I got 16 to 20 thousand caps from doing that. "

You can farm and then sell high level gear from Enclave outposts (which respawn every 24 hours or so).  Plasma rifles in decent condition can be sold for 1000+ caps each with good barter, for instance.

where is there a trader with over $400 on them... all of em that i find dont have but 300 max.....

I sell missiles. They were never really effective for me.