This is a disaster of a name. No one says "cars on the auctions"
CarAuction.com, or CarAuctions.com, would be the perfect name. Slightly worse is CarsOnAuction or CarsAtAuction. But there's definitely no "the" and "Auction" is singular.
I wouldn't put much stock in the freevaluator.com tool... it might as well be a random number generator.