I was looking into some old bikes on Craig's list, for some small winter projects. What book/website you use to figure out the value of the used bike?
Also, is there any "line" you'll never cross to buy used bike? Like, e.g. never buy, regardless of brand, bike older than 1990? Or never buy bike with more than 50K miles? Of course, it's not gonna be the rule, but rather an idea. let's say if you cross that line, you'll most likely get bad deal.

kbb.com or nada.com

+1 Serow

That means you'll need an exhaust system and it's been dogged hard. Might be valve issues from sucking cold air up open pipes. General engine wear is probably more than the norm for the miles.

I never buy bikes that show wrench marks or have other indications that the an amateur has worked on it.  If I am looking for a daily rider I want to see one that has low miles and been untouched, no evidence of any work other than routine oil changes.

I also listen for engine noise and even if it is just some noisy valve type noise in the top end I pass on it.  If the owner says something about oil changes that isn't right then I pass on it.

Also look for bright steady lights and watch the exhaust for blue smoke, check for oil leaks and all those sorts of things.

If I am looking for an old classic for restoration then I mostly want to see if all the expensive parts are there and don't care too much about condition since it will all be torn down anyway.