I bought a Sig 551A1 SBR a couple of years ago, and I have yet to shoot it. I've been thinking of selling it to fund another toy. Is the only way to sell is to sell it to a dealer? Or is it perhaps OK to arrange a sale and then go to a Class 3 dealer for processing/holding?
You can sell it to another in-state resident and only pay one transfer tax, no dealer needed. If you sell it to an out-of-state resident, you'll have to pay a tax to transfer it to that dealer and then another tax to transfer it to the buyer.
VUPD nailed it. It's a bit tricky but it's little different than any regular FTF. The only difference is that you hang on to the SBR until the stamp comes in. For this reason some may not be comfortable paying you for the rifle and relying on you (an unknown) to hold on to it for six months without shooting it, damaging it, etc.. I've sold two like this and allowed the buyer of the second rifle to pay half up front and then the other half when the stamp came in.
I'm not familiar with your rifle so in not sure what it would look like, but you can also have an extended flash hider pinned and welded to it to make the barrel length over 16" and sell it as you would any normal rifle. It's advised to let the ATF know to remove it from the registry, but that's as simple as mailing a letter to them. Then you can sell the rifle face to
face or ship straight to an ffl in another state. If the buyer wants to sbr it, it's an F1 vs an F4 and the same $200 for them.