I would hold onto them.
You really are going to take a bath on the rifle, and what if the next mass shooting is a guy using a brace? Those are on just as flimsy legal ground as bump stocks and binary triggers. And why even sell the can? You can always use it on something else.
This. They are already yours with stamps paid and registered. People are getting frustrated with the transfer process with is killing sales on suppressors. The with pistol braces, hardly anyone is going the SBR route. The only plus to the SBR is having a vert grip vs an afg, and needing permission to cross state lines, and the stamp, of course. Id just hold on to it. Grab you a pistol lower and throw the DD upper on it if you have a need for a such thing. I wanted a MK18 for a long time. The SBR I could get new locally for 1350, the only pistol they wanted 1800 for. Said screw it and bought a BCM pistol and feel I have just a good weapon.