My neighbour used to sell a few motors through eBay, but has had a few problems with "tyre kickers" who win, then turn up and try to haggle the price down.
He did say that selling prices were higher with eBay but there was a lot more opportunity to add photo's & descriptions etc, although the fees were more favourable on Auto Trader, and he was better positioned to play off two or more potential buyers within a few hours of the magazine going on sale.
He's moved away from eBay now as he has his own pitch.
I know a motor trader, who uses autotrader to buy the cars & then ebay/pistonheads to sell them.
I think ebays best for capturing interesting leads when selling motors, & completing the sale away from the site rather than completing the sale on ebay itself.