Best bet is to learn to recognise good quality or desirable parts. This way you can part out a worthless wreck, even if it is beyond repair. I do this regularly, it is one of my best consistent sellers. It helps that my day job is as a bike mechanic. If the parts say Shimano deore on them there is a air chance the bike is worth effort. Old Raleigh parts are also worth money in decent nick, but I mean more than 30 year old. even the little wheel nuts.