So a bit of info about D3's plans for trading/auction house came out from Gamescom:
Someone who plays WoW clue the rest of us in. Is all the trading there for gold? I know there are semi-automated vendor type things, and you can put items on them and specific your price. But when it comes to trading top end items, is that only for gold also? (Most high end stuff is BoE or BoA, but some trading must go on?)in.Diablo.d3: Trading: Very big part of D2. Players like to trade item for item, rather than just selling for gold.
Jay Wilson: We’re looking to improve trading. Not just chat channel spam or creative game names. We haven’t determined the system yet.
in.Diablo.d3: Can we trade item for item?
Jay Wilson: We’re going to do something. Haven’t decided yet.
Why would it be so hard to implement item for item anyway? You've got something to trade, you list what items you'll take for it, and if someone sees your listing and agrees, then they use an interface and get 'er dun.
The hard part would be finding a trade item from someone who didn't wildly overestimate its scarcity and utility. Back in the early days of D2C, when trading with humans was possible before bot spam ruined everything, I seldom found it worth the trouble, since every dude with like, War Travelers, thought they were worth an Un'id'ed Stormshield. Etc.