It's illegal. Trading Standards will prosecute. But a purchaser has the same rights against a trader posing as a private vendor as he does against a bona fide dealer. The trader remains liable for any fault that could have been present or developing in the vehicle on date of sale for six months from the date of sale. Pursuing these scumbags through the courts isn't so easy, though. Even if you get a CCJ and a High Court Sheriff's Order, they may claim the cars belonged to someone else and that they have no assets.