I think they should remove the ability to use real money and make earning credits a bit easier, but PD have dropped the ball lately on both fronts unfortunatelyA 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.