If you're buying it because of what it is, and the price is low enough it's probably a worthwhile gamble. Good headsets are cheap enough that if buying for function alone, it's not worth it, but classic S/R headsets aren't cheap (if you can find them).

You might ask for close up photos to assess the condition, especially one showing the ball track on the crown race. A continuous dull track is good, a series of wide areas looking like beads on a string indicates severe wear and there's a good chance the steering will "index". OK for a show bike, but not for riding.