simple, ask him to go out shooting and watch him shoot the gun. Is it a smoothbore or a rifled barrel wth rifle sights? Either should be capable of keeping them on an 8" paper plate at 100 yards and perhaps a bit more with: 1) the right ammo, and 2) the right guy shooting. Lets see him shoot it and if it works, buy it!!
and old saying, put up or shut up. If he balks at going shooting with you you will know something is amiss and offer him less. Tell him you will pay the full price he is asking if he can do with the gun what he says he can, but be nice when you make that offer, he may take less to not go shooting....<><....
I am thinking it is a rifled barrel because sabots usually don't shoot well at all out of smoothbores, and if it is a rifled barrel that accuracy is possible if he can shoot, the gun is up to the task, for sure....