Hmm, this scam happened to a friend of a friend. He was selling his M3 in Autotrader for a fair market price. Someone who rang asked why he was selling it on eBay for about £5k less than the advert in AutoTrader.
Confused, he found the add on eBay - same pictures as he'd used on AutoTrader. After some gentle persuasion, eBay pulled the add.
What happens is that someone will pose as a buyer of the car responding to the advert in Autotrader. They then come round to your house, so they know where the car is, and if invited inside, they also get to case the inside of the house for future reference....
They then make their excuses and leave. They stick an ad on eBay using your Autotrader pictures. If they get a taker or a definite sale, they then come round and steal your car - remember, they know where you live, and know the inside layout of your house - and probably where you leave your keys as well.