I want to upgrade to a flat top.

I've got an A2 carbine, with gooseneck carry handle mount and Sightron optic on it. The rifle is only three months old. (should've bought the A3 to begin with.) I might try to sell it at a gun show-- just wondering what I can expect to get out of  it. I'm thinking 20-30 percent below new retail.

At a store or gun show I usually can get 50-60% of what I paid for a gun.  Sometimes a little more if it's really mint/new.

Why dont you just buy an A3 upper from Bushmaster.  Then have two guns in one or sell your A2 upper at the gun show better chance of a fair deal.

Your weapons probably worth $650-$750 depending on condition and your area. That's around what they go for out here.
You would make out better selling it local or on here in the EE . Look around at the show and see what new AR's go for.  In my part of PA at the show they for between $855-$990 new.

Tailgunner does make a good point , sell the upper . You might get close to a even up deal for the A3. Good luck