Ken Thorne Cars is set to open a third World of Cars superstore later this year as it embarks on a 12-month expansion programme.
World of Cars, which started trading in Cardiff in January and opened a second outlet in Swansea last month has earmarked a site in Bristol. It also plans to expand further down the M4 corridor to Swindon or Reading next year.
Chairman Ken Thorne said: "The market for quality used cars was ready-made, especially with all the hostility towards buying new."
He dismissed suggestions that the M4 corridor was too small an area to sustain four superstores.
"Eighty per cent of the cars sold from our Swansea site go to customers from the Swansea area, similarly with Cardiff. There is no overlap - we are hardly touching the Bristol area," he said.
The five-acre site in Cardiff, which accommodates 1,500 vehicles, is selling more than 1,000 nearly-new and used cars a month, while the Swansea outlet is on target to reach 500 cars a month.
Ken Thorne Cars anticipates a £150m turnover and 12,000 vehicle sales this year. It owns a Chrysler/Kia/Daihatsu new car showroom in South Wales and a solus aftersales facility in Cardiff, which handles business from both divisions.
"We offer used car buyers a full service identical to new car operations," said Mr Thorne.