"Asda had some success selling cars years ago but Sainsbury's might be overtaken by events," said Prof Rhys, who chairs 'Motor retailing 2000', Automotive Management's autumn conference this month.
Sainsbury's had decided to dispose of its Home Base division because it was outside its core market. He thought intense competition in weekly supermarket shopping might force it to concentrate on its battle with Tesco and others.
Prof Rhys and conference speakers will consider likely changes in UK motor retailing over the next five years.
Eighteen months of intense debate over prices will be followed by Block Exemption review in 2002 and competition from new entrants seeking a share of new car sales.
Prof Rhys repeated his belief that franchised dealers could withstand the offensive from newcomers like Virgin Cars whose chief executive Ian Lancaster is also a speaker at the conference.
"Dealers can offer retail buyers a complete package," he said. "Newcomers to a market don't necessarily succeed. If dealers do their job, they can win."