Didnt realise they took that out. The auction like many forza features (both sport and horizon) has slowly gotten more crap
Its the ability to set your own prices/quantities of tunes+paints that they've gone backwards on. I can still find things that a single user has made but the days of selling cars as if they were an actual dealer are gone, its just Share (or not) your tune/paint for 100 credits a time (plus 500 if someone 'likes' it
)
The Auction is worse than ever, very laggy so I don't bother with bids just BuyOut, searching is still slow and things that modern auctions have had for years like saving searches, auto bids, alerts just don't exist. Oh and if you sell something they now take 15% of the price as comission!