There's already many sites for barter of goods.... why not a site that focuses specifically on bartering webmaster services and e-goods like domain names?
One thing i've always wanted to see is an "open index" some people donate some cash and servers and someone sets up a crawler. Makes the db open so anyone can create their own search engine. You can make the algos how ever you want the db just holds the info/content about the sites and individual pages. I think that'd be really cool and useful. If that existed i would create a search engine where you can search for html tags. I'd call it spamagine
Plus it would really give the top 3 a big run for their money. They've needed a reason to worry for too long now. Open source is the only way to truly get them to shit their pants.
That's another idea... why not build the barter platform off a cross index of all the buy/sell/spam sections of many message boards? Similar to a boardtracker just for opportunities/buy/sell/spam/barter ?