If the car is modified then probably the best place to start are the forums, those in the know who will possibly be looking to modify anyway will quickly be able to see how much money and hassle they can save if they want a car with those mods. They can also check some of your old posts to see if it has been a bucket of trouble and you can check up on them to see if they're bell ends or not.
I've never bought a car from a forum mind but have bought and sold loads of bits, met some real decent guys in the process too.
I'd avoid ebay unless desperate. The car and mobile phone sections of ebay seem to attract the lowest scum on Earth and although these people are a minority, they're a very very annoying minority and not worth the hassle of dealing with.
Pistonheads will attract similar people to other forums, car people in other words, so well worth a punt.
Autotrader still biggest and best, but I'd definately want to sell a fairly standard car on there as it is open to such a broad range of people, most of whom will run a mile if they spot modifications, you'll also attract pond life from there similar to ebay, just not in such great numbers.
As above, watch for the scammers. Keep things simple and if you smell dodgy tell 'em to do one (politely)!
