FFXI, for all its faults, has a good system. As well as the auction house, which would work quite nicely were it not for the artificially inflated prices, you can have a personal "bazaar". You can set prices for items in your inventory - if you have set any prices, a little "bag" icon appears over your head indicating that you have items for sale. You can also add a comment to your player which appears when people examine you.
This does lead to literally thousands of players standing around doing nothing but selling items, but it is a nice idea in my opinion.
I like the idea of an NPC you can drop goods off with, set a price and he will then sell them in the manner of a normal merchant, with the resulting profits being sent directly to your character. To prevent these getting overloaded quickly, there could be individual NPCs for different varieties of items - e.g. an NPC for player weapons, player armour etc. Alternatively, this system could be integrated with the existing NPCs by adding an extra tab to their interface.