I agree, almost any old Smith is repairable within reason unless torched in half.

I can't guarantee this is the problem and several good possibilities have been mentioned, but in my experience, if the thumbpiece release bolt for the cyl is not returning to the rear when the cyl is closed, it will exhibit your exact symptoms. The reason could simply be a gummy, dirty, bolt or more likely a sticking front barrel lug locking bolt at the front end of the extractor rod. If the front bolt sticks or the rod is bent or not engaging the bolt completely (extractor rod slightly unscrewed*), the thumbpiece bolt is not pushed to the rear sufficiently to release the interlock with the hammer, which locks the action.
* NOTE: Model 17 right hand thread; model 17-1 left hand thread.

Very simple to diagnose. When the action is locked up, check that thumbpiece is all the way to the rear. If you determine that is the problem, negotiate the price and it's a do it yourself repair when you get it home. Or if the price is "right", just take it as is!

__________________
Jim
S&WCA #819