MEXICO CITY, Sept 14 (Reuters) - Mexican billionaire Carlos Slim, the third-richest tycoon in the world, may sell his loss-making U.S. retail business CompUSA, a director of one of Slim's companies said on Thursday. Jorge Serrano, director of investor relations of Slim's conglomerate Grupo Carso (GCARSOA1.MX: Quote, Profile, Research), told Reuters one company had shown interest in Dallas-based CompUSA, which sells computers, televisions and other electronics products. "It is being evaluated," Serrano said of the possible sale of CompUSA, which was bought by a unit of Carso in March 2000 for $800 million. "Different alternatives are always being analyzed to...