How about if we look up what fee the seller is posting as compensation for using a credit card and then asking for the manager and negotiating that same percentage as a discount on the purchase price? I ask for the manager all the time and everywhere! I have a saying; "Everything is negotiable. Everything!".
And it's not just car dealers and flea markets. I've talked grocery store, J C Penney, and even Walmart managers into reducing the price listed on their goods. Sometimes it is a day away from being out of date. Sometimes it is opened or missing packaging. Sometimes it is just higher than a local competitor. It doesn't really matter why. You just have to let the manager know that you WILL buy something if they agree to a slightly lower price. Anyone trying to pad their sales using this credit card processing excuse should get the same treatment. If we just join together and refuse to pay it, I don't know why this wouldn't work.