Well there is no such feature.
You are only able to charge a fee based on a price to place an ad. That part of the procedure is totally up to you and your control of how you set things. Once a seller places an ad the procedures for selling something is in the handles of that seller not you.
Payment for an ad is between a seller and a buyer. There is a feature where the seller can allow the buyer to buy it now and pay them via paypal through the program but payment for alot of ads are really through whatever way the seller sets up to the buyer and in alot of cases outside the actual program.
I really do not know of a way to do something like that since whatever site something like that is created for would have to have ever single seller be part of some centralized payment method and your talking about some type of two way payment procedure. You have a buyer that is going to be sending a payment to a seller and you in return are looking for a payment from the seller back to you.