I would say for the OP to try to avoid doing this as a common way to obtain payment, unless they also are going to obtain an FFL. I would venture to say if this is a regular way for payment it would be in the business of dealing in firearms because at some point the OP would have to sell them.
Its one thing to do it once every now and again, but totally different (to me at least) to do it as a common and regular thing.
Also, to the comment about the contacting the police to find out if a firearm(s) is listed as stolen...At least here a person needs to take the firearm to the police department and meet with an officer to have a check run on it, and also the firearm(s) are subject to an immediate hold put on them if they do come back listed as stolen. Its not common here, but is done if an officer is available usually. Its not totally that different then a pawnshop which (at least around here) is required to provide records to LE and if an item is reported as stolen matches the description it can be held by LE until it is sorted out.