I kinda regret getting rid of my Contour, due moreso to the memories associated with it rather than the car itself (though I do miss the car!).
One possession I regret getting rid of the most was my Colecovision console and games - although that was by no means my choice (I was 12 and parents donated it very much against my will). And my SCSI cable for the old Mac's external hard drive. Though I can repurchase both for under $200 so it's not necessarily a legitimate regret, just difficult paying good money for stuff you knew you had before.