yes thats a really good deal.
for my full 400w MH setup i paid $250 brand new.
so certainly used $50 is a great deal. he could probably get at least $100 for it but hey dont tell him that
and of course find out how old the bulb is in it. and how many hours it has been used and look at it to make sure theres no cracks ect. if it hasnt had too much use its probably okay to keep using it, but I would say buying a new one is a worthy investment if you can afford it. I think they are typically about $50 for the bulb depending on the watts. at least thats what it is in my area. im sure its different everywhere.