Further to Falconflyers comments. I am looking at a 20 gauge shotgun to buy right now. Less than 3000 ever made by a guy widely regarded as making the finest boxlocks ever produced. The majority I see are 12 gauge. A normally small number are 16 gauge and a few 10 gauge show up now and again. In 10 years of looking in the US and Canada I’ve seen exactly two 20 gauge. They are both high grade versions, one higher than the other, and they are currently listed for sale on GI for $42,000 and $27,500. Listed, not sold.
The interesting thing is the less expensive one, identical to the one I am negotiating on, sold at auction 5 years ago for $9,200. And when I contacted a collector friend who may have more of these guns than anyone, he thought the range estimate I should pay, for a gun identical to that $27,500 gun, was $8-$12K.
If I were you, OP, I’d use the gun. They are fantastic OU, if the chokes suit what you are hunting. But if you do want to turn it into cash, accept what the market tells you. BTW currently it’s about $400 and 3 months plus to move that gun into the US.