This one's a no-brainer, if it could be implemented well, and it's truly a "killer app." Adapt your client to handle online auction processing. There's nothing that eBay does that you couldn't do. All eBay does is provide a forum for exchange of information, which is exactly what P2P is. And with eBay constantly raising its fees and squeezing the sellers, there's be a mass exodus from eBay if there was a free alternative which worked.