It very much depends on the car. If it's a modern, readily available car, I'd probably be cautious. However, it's a great place to find some rarer cars: cars that you'd normally have to travel to find, and cars where it doesn't make much sense to advertise locally (you need a wider audience than that!).
It also depends on the advert on ebay. If there is a long, detailed discription, listing good and bad points, and lots of photographs, then you should be OK. In the UK at least, if the vehicle doesn't match the description, then the seller has misrepresented it, and you don't have to buy it if you win the auction. However, if you do win, and the advert is accurate, you're legally obliged to complete the transaction.
Avoid short adverts with little description, or ads with small and fuzzy pictures. And always check out prices for similar vehicles, to make sure you're not paying too much. And never pay until you've seen the car!
I bought a car on ebay recently, and I'm very happy with it. I think I got it a little below its market value, and it's fairly rare, so I wouldn't have found one locally.
And check out their feedback! If they've sold a lot of cars in the past, walk away! They're more than likely a dealer, but without the legal comeback for warranties, etc!