I haven't found to many sellers that take gunpal on gunbroker yet,or at least the ones I have ran into, but I hope more get it and advertise that they accept it. I don't like paying 5 dollars to a bank and waiting twice as long to get my item.
another advantage for sellers that i just discovered is that the buyer doesn't have to 'sign up' for gunpal in order to pay you through their service. when your auction or sale or whatever is done you (the seller) can go into your gunpal account and create a button/link for that particular item at the agreed upon price. you can then email the link to that 'button' to the buyer who can click it and follow the instructions to pay you. the link takes the buyer to a secure site where they can either sign in to pay with their gunpal account (if they have/want one) or just enter their card info right there and pay you that way. pretty slick feature that i think will come in handy for gunpal users who are registered as merchants. not sure if this feature is available for non-merchants, though.