That's between the buyer and seller to work out.
Actually, agreeing on a transfer fee is between the buyer and the FFL. The seller has nothing to do with this part of the transaction. The way the typical Gun Broker deal works is that the buyer pays the seller the price of the gun, shipping, sales tax, etc., and supplies the seller with the name and license number of the FFL to receive the shipment. When the seller ships, his involvement is over. In the meantime, the buyer has worked out the arrangement with the FFL, including the fee that will be paid.
Gun Broker maintains a data base of FFL's, listing their locations, hours, and the fees that they charge. It's therefore easy for buyers to comparison shop among FFL's.
Police departments don't have FFL's and don't do firearms transfers.
In Virginia, the State Police act as the point of contact for NICS checks. They also are required by law to set up tables at gun shows, to run the required background checks for transfers between unlicensed individuals.
The OP may be referring to a similar situation in Maryland. The OP is unclear whether he's talking about a mail order purchase through Gun Broker, or a local face to face transaction. It may well be that if this is a FTF transaction, the parties could go to the State Police barracks and have them run the background check. I'm not that familiar with the Maryland procedures.