It can go either way, don't forget that if you're buying you're looking for a bargain, if you're selling you're looking to make money. Generally its a buyer's market.
If you sell you have to pay eBay a percentage of the final sale price, about 10% I think.
Agree with Skids that it is generally a buyers market. I also think it depends on your situation. If you want a fast sale then ebay is quite a good option. If you want a good price for a car and you are not in a rush to sell I would suggest an advertisment in the paper/motoring magazine. Ebay has a huge potential audience which is another benefit.