Looking around I've seen that used rifles by the likes of Parker Hale, BSA and BRNO can be had for as little as £100 even in shops, not just privately. Question is, how cheap does a rifle have to be before it's probably not worth buying? What can be wrong, and how hard/costly are these things to put right? How does the untrained eye like mine look at a rifle and make an assessment (obviously I'd try and take an experienced eye with me, but before I drag somebody out if I want to make an initial decision).
In short, when I see a rifle advertised at £100-£150, how do I tell if it's too good to be true?
Please use simple language, I don't know the terminology!