You reply to what looks like a genuine inquiry (not the normal Dear Myster, I want your item for a birthday gift for my son who is on a top secret mission in the North Sea - cut and paste from an actual email) and the potential buyer says they have looked at many cars and ask if the URL they send you is your car advert. The link takes you to a very convincing webpage which looks like autotrader account login page but it's an attempt to harvest your login details.