The dungeons are actually not all that necessary, if you give people good prices (start at 104% and feel out what people are willing to pay from there) you can build long chains of sales that give you a ton of experience, and leveling up your item shop is how you actually make money. I suggest running the dungeon a few times for cheap product to sell, but pretty soon you're better off just playing the market. Keep lots of stock and when the price of something goes up sell all of it you can, when the price drops go buy all that you can.
HoChiWaWa's forum posts
The hint on the last page of this thread is pretty sufficient for 6, although I'll say that it DOES have a colon in it.
I'm lost on 5, the hint on the last page for that one made no sense to me, I couldn't even parse it.