John Strip Motors in Llanbradach, Caerphilly, was fined £750 and ordered to pay £392 costs. Mr Strip has since closed the business, but is thought to be trading in another area.
The case, which was brought by Trading Standards officers' under new powers to enter and search garage forecourts, was at court for the second time. Gwent magistrates threw out the original case, ruling that Caerphilly County Borough Council had brought the prosecution under the wrong regulations.
However, the High Court upheld the right of Trading Standards to enter used car forecourts in search of potentially dangerous vehicles and ordered the case back. Tony Allen, Trading Standards Institute research and development manager, said: “This is a victory for the rights of consumers to be protected against those traders who sell potentially dangerous vehicles.”