I thought about (and setup a prototype) starting an ebay competitor a few years ago now.
The main thing it was going to do differently, was instead of having a fixed ending time for an auction, it would be conducted like a regular auction - in real time, via AJAX, and the auction ends once no one else bids.
Seemed like that realtime element, along with no 'sniping', fairer bidding, a cool UI etc would be a good reason for people to switch.
But the network effect is massive. To be successful as an ebay competitor, I think you'd need a pretty large marketing budget, or several people working on promotion full time.
It's ripe for the picking though. And massive profits.