I can see why an auction wouldn't take cards.
It's necessary in retail. The bike shop didn't take cards when I started, and we ended up writing down the stuff we didn't sell because we didn't take cards. Once we were setup, cheaply since we already had a machine of sorts, it was ok. Basically more than tripled our sales. But the deal she had was probably the worst. Something like 3.5% plus 35cents per, plus if we did less than $50 in a month there was a $50 fee. Not great for a seasonal business. One of us would go in once a month from Dec-April and charge ourselves $50 for something, I usually bought parts, or cash. At the time Paypal was actually much cheaper.
(And Amex charges way more. we've never taken it)
We started by having a $10 minimum, but a lot of people don't have enough cash when they're out riding or jogging. Unless they weren't at all local, I'd just let them take a water or soda, and bring the money the next time. Especially in the summer.
Nearly everyone did, some even gave us a bit extra.
Unless the auction figures they'll make more by taking a card it just doesn't make sense.
Steve B