Re: buying a used rifle with issue...
I once blew an ejector with a hot load. Needed to tap on the bolt handle with a hammer to get it up. Might be something as simple as a damaged ejector or extractor which would be minimal cost. If the bolt is shot, not a bad thing really, the PTG one piece bolt is better and get the mini M16 extractor with it.
Hopefully receiver and barrel are OK, but like everyone else has said, get it checked out by a qualified smith.
You will never know how much it has cost my generation to preserve your freedom. I hope you make good use of it.
~ John Quincy Adams