Buyers commissions seem to be around 10% these days. Sellers commissions can vary depending on if the seller opts reserve or no reserve.
Another 5-10% for the “house”.
Then there is generally a listing fee to put the car in the auction to begin with. $500-1500 is typical.
Factor in transportation and hotel fees for the buyer to get the car there, and stay in the auction city.
Hard to see how many sellers are left with any money after the car sells!