Unless you REALLY know what you're doing, only buy from CMP
Thats not good advice, in fact, I think it is really bad advice. I agree that CMP is the best bet, but there is no reason to feel that you can't pick out a decent M1 Carbine without going through the CMP. With a little knowledge, you will do fine. The carbine is not hard to understand.
A few general guidelines would be:
Don't stray into commercial reproductions, and avoid the Universals, Auto Ordance, Plainfield, etc. If you don't know what you are doing, USGI is safer, and are the only ones that will go up in value generally.
Check and see if the carbine is an import. These were guns that were sold or loaned to other governments after WWII and then reimported. Some are in bad shape. They are general marked with the name of the importer on the barrel. Be careful of these. Some are fine, but if you don't know what you are dong, you can get burned on an import, and they are not as desireable as non imports. If you know what you are doing and can find a decent import, it is ok to get it, but it is not worth as much as a non import.
There are very few "all original" M1 Carbines out there. Most were rebuilt several times while in Army possession, and most have mismatched parts. My general rule of thumb is that any M1 Carbine being touted as "all original" is a fake until proven otherwise. For example many people will take a mix-matched Inland, and swap out the small parts so they are all Inland made, then try to pass that gun off as "original". Its not.
Expect the parts on an M1 Carbine not to match, for reasons mentioned above.
Otherwise, assess a carbine as you would any other used gun. Look at the bore. Check the finish. Check the small parts.
There are many many finer points, and they are the subject of many books on the subject. But these guidelines will get you off to a fair start.
