When someone says they just acquired a 5 screw S&W or a pre-war Colt, an Old Model Ruger, a pre-'64 Winchester we all congratulate them.
Most guns are carried much, used little. When I started shooting and collecting seriously in 1971, I asked a gun guru about a Colt Trooper 357, when I mentioned some of the bluing was worn on the barrel he replied:
"You don't shoot bluing."
In a private sale, I have found it's like buying a used car-you don't buy the car,you buy the owner. You go to the house, it if looks Like Tobacco Road or Omaha beach on the evening of June 6, 1944, forget it. If the house is neat and tidy, the people presentable, you'll do OK. I m currently driving a 1986 (!) Oldsmobile Cutlass Ciera. On July 30 I drove to Duxbury, MA and back. 600+ miles, didn't miss a beat. I have guided to people to cars on Craigslist, 20 year old senior citizens car, low mileage, well maintained, interior and body fine. Say $2000 as opposed to $25,000 for a new one. Are you out to impress people or do you just want a reliable car ? For some rifles-the M-1 Garand, e.g. there is enough information on what to look for, tools to check things such as barrel wear, etc.
Dean Grennell said he had an on M1911 with a well worn barrel that fired patterns, not groups. A new barrel quickly cured that. Ed McGivern had pictures of tight machine rest groups fired out of S&W K-22s with over 200,000 rounds through them.
The main advantage to buying new is the factory warranty. I have only taken advantage of that once, the transfer bar on my DW M-12 broke.