Southland Corp., the 7-Eleven store chain that is shedding some assets to finance a $5.1-billion family buyout, may have a buyer for its auto parts unit--the Dart Group Corp. The Landover, Md.-based Dart, known primarily for its discount-store operations, is controlled by Chairman Herbert Haft and his son Robert Haft, president. Dart said it is waiting for a sales prospectus from Southland before deciding whether to bid for the 465-store Chief Auto Parts. Dallas-based Southland would not disclose a potential sale price, but analysts estimate the division's worth at $150 million. If Dart does make an offer, it will be the first announced bid for any of the Southland assets on the block. In addition to the auto parts unit, those assets include dairy, snack food and food laboratory operations. A Dart spokesman said the company was interested in combining Chief Auto with Trak Auto Corp., the auto parts chain that Dart controls. Dart owns two-thirds of the chain, which has about 220 stores.