the auction house is the game
Free game idea I'll probably never implement, and would hate to maintain if I ever did:
Yendor's Other World (or, YoW) - a cross between IdleRPG / ProgressQuest and the Auction Houses from World of Warcraft. The market is the game, the roleplaying is just the background. Characters quest while you're busy in the real world, and you end up with new items and money when you return to play. Buy, sell, and trade items with other characters. Collect sets of armor to pimp your avatar and show off. Buff your stats so your avatar can finish better quests to get better items and start the whole cycle anew.
But, alas, I don't know anything about auctions or markets, and I don't like people enough in the general case to put up with running an online game unless it made me piles of (actual) money and I never had to answer a support email. :)