I am not a lawyer, and this is free advice, so take it for what its worth. This is the typical way these things are handled in my area. Your area my be different to some degree.
This is generally a civil matter, meaning as long as the sale was legal, and he did everything but pay you yet, its basically a dispute of payment between the two of you.
With that said, probably the first thing you could do, is to type up a letter requesting payment from the FFL, and send it return reciept requested. In this letter you want to include a copy of your consignment agreement, and also outline when you were notified that the firearm was sold/transferred, the agreed upon price and fee's, and give him "X" days to pay. Keep a copy of this letter and the reciept from the post office. This way you have a documented attempt to contact him to obtain payment.
After you send this letter, you may give it a week or so and stop by to inquire, but keep in mind, if the business owner or a representative ask you to leave, you should. Just FYI. If you do not recieve payment, from either at this point, go to the clerk of court (may be different in your area) and file a small claims suit.