I never dry fire. Some guns made today might have steel to withstand the snap, or springs that are lighter? But as a rule I just don't do it. I understand many people say they dry fire their guns all the time, and never have a problem. But the thing is you won't have a problem until the nose of the firing pin breaks, and there's no measure of time which can indicate how many dry fires it would take. What people need to understand is it can be an expensive fix if the parts are rare, and a gunsmith needs to make a firing pin. Could be $100 or so? So I can understand if a seller said no to dry firing his gun.
They make snap caps which have a spring pressure plate to allow the nose of the firing pin to hit it, and move forward, safely.
Also, if the gun is a rim fire, dry firing a lot can put a crimp on the edge of the chamber over time, and can cause problems that will need fixing.