Since the gun was a fairly high priced quality double in excellent condition... and the ejectors worked with snap caps and for the first several shots... is it possible the problem is one of dirt or congealed lube? Did your friend have it fixed?
I tend to agree that a vintage double has a finite lifespan, and the dealer has little control over say a spring that may have taken a "set" from being left compressed while the gun was stored. He also doesn't know if the gun was fired in the first week with one box of shells, or was taken to Argentina on a dove hunt where it is fired almost to the brink of the rib joint solder melting. The seller ought to clearly state their return or warranty policy, and the buyer should ask about that before the purchase.
If you sold a low miles 1939 Rolls Royce in great condition that always started and ran perfectly without any sign of engine problems... and the guy who bought it called after a week and said the motor blew up... would you just pay to fix it or give him his money back when it's possible he was drag racing or forgot to put in fresh oil after draining the old stuff out? This is a very gray area.
