In a salon, contrary to junkyards and barns, you will find new cars. New both in terms of condition (100%) and in terms of models - you won't find any vintage cars, such as Bolty - rather something like Mayena. Not everyone will be able to afford such luxury, though. New cars can cost well above 100 000$.
Auction House provides the best balance. The cars there are diverse both in terms of price and age. The prices are pretty random - you have to be the last bidder to win an auction. The estimated price is based on the visual and technical condition of a car (which is also influenced by how complete this car is).