In the last post about the all-dungeon campaign, I meant to add a discussion about inns and merchants in the dungeon. In Nethack, when you explore a level, you may stumble upon a shop, where a shopkeeper will be happy to sell fresh supplies to you and usually will also buy items you don't need. You might need a little more to your design to explain how a shopkeeper (or innkeeper) is able to survive in the middle of a dungeon.
But there's another issue that occurs to me, one that applies to ordinary merchants above ground as well. If you write up a description of a village merchant or use one from a module, the notes generally indicate what items are for sale and how much cash the merchant has on hand. How many of you adjust cash on hand after a player buys something? And how do you account for the merchant's own expenses? Or do you?