I think being able to sell duplicate cars to an “NPC” vendor would help reduce the clutter in My Cars menu. Sure, you can use the auction house to sell them to other players, but cars you have the most of are generally the most common ones, that everyone already has, so nobody will buy them. Horizon 5 added a way to gift cars to other players, but all this resulted in was players dumping excess cars on others, who most likely already had them in excess as well.
Duplicate cars obtained from Wheelspin can be sold (although only immediately when they are obtained), so why not those obtained in other ways, such as from playlist rewards?