You sell a nice gun to BillyBob on THR.
You ask BillyBob to send you an FFL from his receiving dealer. Some dealers won't send private individuals their FFLs, so have them send their license to your FFL guy.
Have a look at the FFL you receive, and go here:
https://www.atfonline.gov/fflezcheck and check it out. The reason a lot of FFLs won't deal with "Joe Unlicensed" is that that some ol' Joes photoshop FFLs and pretends they're an FFL. Checking via the ATF EZCheck covers your butt.
After all the paperwork checks out, send BillyBobs FFL dealer the firearm in whichever method you desire. Handguns must ship overnight, long guns can ship however you like.
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However, I have seen folks who sell guns, look up the receiving dealer on the EZCheck and send it w/o having a paper copy. I am not saying yay or nay that this is for everyone, but that I've seen it done. This method will save a lot of time, if an online payment system is involved.