If the person who is buying a used double rifle doesn't have the knowledge required to tell if a double is what it should be, he is a fool not have it checked out by someone who does know.

If the person you are buying the rifle from won't let you get the rifle checked out, then you need to find another rifle.

I have a small circle of friends who I trust to tell me about double rifles. My friends, Mark Cash, aka 400 Nitro Express, Mac, NE450#2, PWN and JJ Perodeau at Champlin Firearms, Enid, Ok.

You have no one else to blame but yourself if you choose not to have the rifle inspected. Just my opine.