Used and under $100 at a gun shop normally means look it over real careful as they want to get rid of it as quickly as possible.
I'd have the gunshop supply a borescope so you can see things in greater detail.
Also look at the muzzle and the breech plug area for wear and/or neglect.
If you see signs of rust and neglect, I would pass on this weapon unless you were looking for a parts gun.

If you are looking for something inexpensive, for an extra $50 you can usually find a new ML from Traditions, CVA and so on that will also get you into a lifetime warranty and some support.