The obvious problem is how to deal with scammers. IE those who sell an item but do not actually deliver it.
This can be remedied in various ways, E-bay uses a rating system, which is a good idea. The problem is that with the complete anonymity of bitcoins scammer sellers would simply make a new account. Making accounts in some way not anonymous, and in need of verification, could, perhaps, be a solution to this problem.
If lots of scammers are making new accounts then the value of having an older established account with a good rep will go up. Obviously we'll occasionally have to trust newbies to get things started, but you can start dealing with them with smaller amounts. I don't think it's a fundamental problem is all, solutions can be found.
The value of BTC has risen a lot in the few days I've been here. I notice lots of prices that are high now because people aren't used to adjusting downward quickly. Obviously the increase in value will slow down, but in a longish auction you might end up getting bids in the beginning and then no more because the BTC are more deal at the end of the auction than they were at the beginning. I'm not really saying much here, just that I'd expect people to prefer short auction lengths. Of course not too short because you want enough people to see it before it goes offline.