Not to buy, mind you, but to stop riding. I went on a ride yesterday and saw a friend's bike with a dent in it that looked like it had been made by a sizeable meat tenderizer hammer. Is there a way to tell when a bike frame/fork tube has been damaged enough to actually warrant (gasp) the junkyard?

Never- Tubes can always be replaced! The question becomes cost, can you buy an undamged frame for less than the repair?

I like your attitude! However, the question still stands in a slightly different form - how much is too much before you need to get it replaced?

Judment call mostly, any dent in a tube will change the alignment to some degree, a crimp in the tube will would weaken the tube and at that point IMO it would need to be replace or the frame retired until it was repaired.