Although I understand the idea of buying used guns instead of new in order to save money, my experience over the last few years is that used are selling for almost the same as new. I would rather spend the extra $50-75 to be sure that the firearm I am using to protect my life is 100%. I have seen used revolvers which have problems with poor gap at the forcing cone, abused cylinders because the previous yahoo was shoot loads too strong for the gun, and even some "creative modifications" that create a bad gun. Along with that, as others have said, you need to be able to check the ser # to be sure it isn't stolen. Keep in mind that the gun could have been sold legally to someone you know and respect BUT if it was stolen a few owners back, the bottom line is you could have it taken away from you with NO recourse. It is the buyer's responsibility to due diligence to protect themselves.
I also believe that the nicer your defense weapon is, the much great likelihood it will be "lost" in the evidence locker while you are waiting for your case to be cleared. Buy a new reliable brand firearm (Glock, Ruger, M&P) that you can shoot well. A 9 mm +P with a 10-15 round magazine shot with proper aim is better than a 460 magnum that misses the target.
Getting back to a 12 ga pump, it may be harder to practice if you live in an urban area but, I would say it is worth being friends with someone who has a outdoor range membership or taking a trip to the country once in a while to visit a friend's farm. A shotgun require a lot less precision aiming, has less risk of wall penetration, and has enough knockdown power to handle virtually any uninvited visitor. Lets face it, if you live in Chicago, NYC, L.A. or some other less than friendly environments, you have a better chance of owning a 12 ga hunting rifle that a 9mm pistol which will "blow out a lung"
