Several years ago I bought an older used Kurt 6" from a guy on Craigslist. Seller wanted $80 (bargain of a lifetime!) and my timing must have been just right, so I got it.
It was gunked up like heck, but seemed good otherwise. Cleaned it up once I got it home, then bought a rebuild kit from Kurt. The kit was for newer models, so I couldn't use all the parts. But I got many (like the brush that surrounds the leadscrew and keeps swarf out of the nut) to fit well. Unfortunately, the vise was just too big for my round column mill. So I offered it for sale on Craigs (for $300). Mentioned why I was selling it. It got snapped up within a day, and the buyer seemed pretty satisfied.
I do appreciate your caution, Terry ... maybe I was just super lucky.
In reply to Roger, I'd be willing to go as far as to say that Kurt vises are so well built that all you really need to look for is visibly obvious damage and/or rough operation of the leadscrew. Others have given examples of some obvious flaws to look for. If it looks OK and works smoothly, I'd say "go for it!"