Maybe you've never received payment by credit card. Whether it's a credit card processing company or PayPal, they all get their piece of the money for processing the transaction between the seller and the buyer.
I don't know how GunBroker or the like works, but I do know how eBay works. eBay gets their percentage of each sale, and PayPal gets their percentage for processing the payment. It all comes out of the seller's money, which the seller has to realize when that seller prices an item.
The small company I work for has to use a credit card processing company and we used to add that cost on top of the price. We quit doing that since we just hide that cost in the price now. Our prices went higher to cover this cost and NO ONE gets the previous Cash Discount.
Strangely enough, our customers would rather pay the higher price than be given a cash discount price.
There is only one local store I frequent that actually offers a discounted price for using a debit card or cash. I like it a lot and I save a little money.