You may be thinking of eBay as it is today in which only a small fraction of merchandise is even sold by auction.

In early days of auction sites, if you let a bad buyer (one who may not pay) win the highest bid, and it takes days for you to find out they’re not going to pay, then you’ve lost the other bidders who were just below them, and you have to start over and relist your item again and hope to attract bidders again and get as good a price.

A week of delays before you get paid, and risking the payment price or even the whole sale all seem like good reasons to reject bad buyers.