One of the problems (or "properties" at least) of online communities is they naturally evolve into monocultures. So while thing like
http://www.biddingpond.com/ are laudable efforts to get a Bitcoin economy going, they're going to struggle to ever be as useful as eBay until BTC becomes preferable to fiat currency, assuming eBay itself doesn't allow listing and bidding in BTC.
I just put a CPU up for sale on eBay UK and I've offered to accept payment in BTC (in fact, I've offered to waive the postage costs as an incentive, just to see if anyone will bite!) and it gave me an idea.
A useful website would be one that lets you track eBay auctions that are taking payment in BTC. Some sort of convention (eg embedding a certain URL) could even indicate that it is a BTC auction automatically. This would get over the usual barrier of needing to create a community first. It would allow an auction economy to bootstrap off the back off the eBay community. There's a load of technicalities to worry about (how to handle the exchange rate etc), but it may be worth thinking through.
Maybe this idea has been discussed before (if so, sorry for the noise!) but if not I hope someone considers it useful. I'm actually a web developer myself, but I have too many other things on at the moment to have a go at this. I'm hoping someone else thinks it would useful enough to actually go and make it :-)
