Some points to consider maybe
Start doing it like that and the site may have to pay tax on it, when you buy something at an auction it is tax that is added at the end, the seller is deducted a fee for the auctioneer selling the item on their behalf
I think every seller should pay a set fee or a percentage to the site for it's running costs, not the buyer who is taking a chance with the sale, the seller has got his ready's.
If this is implimented that the buyer pays the comission, what would happen there is a dispute in the sale would the site have to refund the fee
Also auctioneers hold the money untill any (if any) guarantee period has passed
Maybe a bigger head ache than you think, I understand where you are coming from Stuart