I've never heard of this either. I would think that if it is in fact a swollen hock. (not doubting, just don't know for sure) then it would depend on how swollen, and how often. Is it swollen enough that the horse is lame? How often does it happen? What excerbates this? (these are questions I would have for the present owner) I would definately get a vet check and during that ask the vet about this. Does the current owner have any xrays? Why is she/he selling the horse?
It may be nothing, but since I don't know that much about it, I would be leery.