Ebid is legit, so if you find stuff there, go ahead and buy. But eBay is the defacto auction site, so most people are going to be using that.
There are some bid sites which end up selling stuff for cheap, but each bid costs you a credit, and you buy credits from the site for real money. So it's going to cost you anywhere from a few cents to a dollar to just place a bid. Those are the one's I'd stay away from.
I'm sure that someone out there might get to be popular and upset eBay as king. We saw Facebook do this to MySpace. But until someone makes a service which is a lot better than eBay, it's not going to happen.