People are asking why you would want to know how many rounds have been put through a used gun. Presumably you'd want to know for the same reason you'd want to know how many miles are on a used car. That is to say a round count would give you some sort of information about worth, and/or remaining life regarding the firearm. This makes little sense to me. Firstly, there is no law saying you need to keep track of how many rounds have been put through your gun unlike miles on your car (odometer laws). Secondly, and unlike cars, the useful life of firearms are rarely, rarely reached. Per Hilton Yam you can expect to retire a 1911 after 70,000 rounds or so. After 80,000 rounds through my first 1911 I would tend to agree with this (if you are shooting full power loads. If you are shooting reloads expect 100,000+ before cracks in the frame *might* develop [http://www.10-8performance.com/pages/Reliability%2C-Round-Counts%2C-and-Longevity-in-1911s.html]). My point being most guns will never, truly, be "used" guns. Even polymer guns can endure thousands upon thousands of rounds before needing severe attention.
As such, "used" guns are sometimes great deals.