If the machine is in great condition and hardly used £170 might not be bad, but I personally wouldn't go that high considering it was only £200 new and that was with a guarantee which you won't have. It also matters if you know the person who is selling it and can trust what they are saying. if you don't know them you have to look at it realistically and not pay over the top for it. Good luck with whatever you decide.
Peter
Peter