Sometimes I found cars who belonged to celebrities or sportsmen in classifieds too, I remember seeing ads for Renzo Arbore's former Jaguar XJ or Vincent Candela's former Mini Cooper (well, less famous worldwide than Rod Stewart but still.. ) and surely others too usually more valuable cars that belonged to world famous people end up in auctions too, like Diego Armando Maradona's Ferrari Testarossa or one of Michael Schumacher's personal cars (IIRC a Mercedes W210 AMG)