I dont think the law applies to private sales. Mine are always parked on what is part of my garden.I think it depends on the local council's attitude, ours got fed up when the same guy was parking ½ doz or more cars along the road with for sale signs on them. Police put unauthorised trading signs on them saying they would be removed in 7 days.
It's legal if you get a street traders licence for it but I don't think you would have much luck with that.