It all depends on the condition of the gun. Some can be shot, and are. Others are wall-hangers. You generally get what you pay for. If a Greener were in good condition to shoot, it would be expected to go for much more than $300.00. And, then, the owner would not shoot it!
On the other hand, this may be a Damascus steel barreled gun, which until very recently, folks were told to never shoot. The "Common Advice" was that any Damascus Steel barrel was too weak and unreliable to trust to shooting. Recent testing, with results published in the Double Gun Journal these past couple of Quarterly issues, indicate that the Damascus steel barrels actually can take much higher pressures than previously believed, and that even those with obvious rust can be shot, and repaired.
Who'd A Thunk it????? :idunno: