I've done about six deals on U.S. Audiomart. All but one was F&F. The one that wasn't was a dealer listing. I would never do F&F unless a seller had excellent feedback or if the dollar amount was small. A few sellers were willing to do standard PayPal goods and services if I paid the 3% PayPal fee.
I see the same thing on Gunbroker. Many sellers add 3% if you pay with a credit card.