I think that some FFL's who are in business have had experiences that some of us don't know about. If you were an FFL, and a gun you got in the mail turned out to be stolen, would you rather have received it from Joe Schmoe the ragman, or from an FFL? What do you think are the relative probabilities of receiving a stolen gun from one as opposed to the other?
An FFL can pass along a stolen firearm just as easily as a nonlicensee. FFL's have no magic system to check for stolen,,NCIC is secured for L/E use only.
FFLs buy from non-licensees all the time,,either at their shop or at gunshows. The used gun goes up for sale on the net, it gets shipped by the FFL.
The FFL doesn't provide any guarentee to the transaction to insure that the gun isn't stolen.
If loosing money by having a (stolen) gun confiscated is a concern,,in the end, it doesn't matter which source it came from. If the gun is shown to be stolen, you the FFL will loose the firearm.
You will not get instant compensation from either an FFL nor a non-licensee that sent it to you.
Both will have received the gun from someone else before sending it to you and the finger pointing starts, but no check writing begins.
The restitution and the crime are two different issues,,neither quickly nor easily sorted out generally.
As long as Mr Schmoe has the proper ID that you demand of him, you keep it on file, you log the firearm properly in (and out), you have nothing to fear of the transaction from the BATFE.
Of course if your suspicions tell you that the gun may be something other than what it's being presented as, then you have the right to refuse the seller/buyer and the transaction.
As long as you follow correct procedures (obtaining proper ID of either the FFL or the non-licensee, booking the gun in and out, etc) you are just another person in the line of possession paper trail.
But as always, the FFL has the option of setting his/her own criteria, and if it makes them feels better rec'vg from another FFL only, then they can set that as a rule in their business.
What ever they want to do. It may hurt their business as others see it,,but when you have an FFL you get protective of it. Especially in todays world.