Ian Graves, of Southcote, Berkshire, breached an injunction stopping him selling cars without telling buyers he was a dealer by placing adverts in trade papers, Reading County Court heard.
Graves claimed he had misunderstood the injunction, served on him last year following a two-year council investigation into his use of public roads to sell cars.
He used to park cars bearing a 'for sale' sign and a phone number along a main road.
In court, Graves said he was not a trader because he made no profit on sales and that cars were just a hobby.
But Judge Ann Campbell dismissed his definition of a trader after hearing from trading standards officers that Graves put 13 adverts in the Thames Valley AdTrader between September and December 2006 that did not make clear he was a dealer, and therefore subject to consumer protection laws.
Graves, four months into an unconnected two-year jail sentence for handling stolen goods, was given a further six months after the judge said the evidence against him was "comprehensive".
She said: "It's very important that the public have the protection of this legislation, and you intentionally misled large numbers of people."