If youre buying from a private seller, or anyone else for that matter, look down the bore and at the part of the op rod you can see in the front handguard when the bolt is closed and as you pull the op rod back. If it looks dirty, dont buy it unless you run a patch through it and the bore looks good.
Ive had two in the past that looked "dirty" when bought (for some reason, a lot of people don't like to clean their guns
), that turned out to have been shot with corrosive ammo and put away dirty for what must have been an extended period of time. Those dirty bores never cleaned up, and both guns were shooting around 12"-15" at 100 yards.
Both guns looked great otherwise too, with nice wood and decent finishes.
I also had a 308 match gun that must have ended up being re-barreled for a similar reason. The op rod showed the same/similar corrosion the others had, although it wasn't quite as bad. That gun was a shooter too.
There are lots of good guns out there, and probably an equal amount of not so good, for various reasons and what your eye might tell you.
If you get one, get yourself a SLED and understand what it is and why you want to use it. The M1 is a good gun, but it also has its own little quirks you really need to know about and understand. Especially if you reload. Pay attention and do your homework.