A direct person to person selling scenario would probably lead to Real Money Trading, Joe Bloggs doesn't want to grind 400 races over 8 hours for a Shelby Deytona so he pays Joe Soap to do it for him instead. At less than the cost of in game MTX.
Also, in a scenario where there is an auction house type deal, Joe Soap would just buy multiple high end cars and list them once they are gone from the Legendary dealship for X% mark up to corner the market.