Generally your item will, more or less, fetch the price the market is will to pay for it. Buying a used car tends to be a bit of a lottery anyway. Even buying one from the trade can have problems. Sure, you can take it back if there's a problem, but the extra price you'll have paid probably includes the "Used Car Warranty".
From an eBay sellers perspective, the days of standing arround, watching your buyer kick the tyres and suck teeth before trying to beat you down on price are over. Your buyer has already paid for the car. He arrives, signs over the registration document, takes the keys and drives off.
Now, obviously, if he drives around the corner and the bottom drops out of the engine you will have an issue. Even if you have put "Sold as Seen" on your ad you'd have to consider carefully whether you wanted to continue as an eBayer if you wanted to invoke that privilage. But then again, if you had an inkling that the car was going to suffer from a catastophic mecahnical failure, you'd either put that warning in the ad, or not put it up for auction in the first place - wouldn't you?
Happily though most modern cars either work or they don't. So the first turn of the key will tell you if things aren't right.
From a buyers point of view. You have a wide selection to choose from and you can check previous sales for price and spec comparision. Even so, if you are a first timer you might want to take somebody with experience with you but remember - you've already bought the car "Sight Unseen".
However if the car looks nothing like the photograph, or major damage hasn't been shown. You should ask for a refund, walk away, and then consider negative feedback. If the tyres are bald, you'd be breaking the law driving the car away. And remember the point I made above, if you're considering spending time looking under the bonnet and checking the oil filler cap for emulsion deposits (signs that oil and water are mixing) to get a price reduction - you shouldn't be buying on eBay in the first place.
I've sold two cars on eBay, my beloved Saab 900 Turbo and a 20 year-old Renault 5 that belonged to an 88-year-old uncle. Both were seamless, worthwhile experiences.
Why? Because eBay trading is all about shared trust and honesty. That's why I do it - and that's why you do too.
Happy Bidding and Selling!