If you count the shaft cost and a case, $850 sounds like exactly market value, but no bargain. It's a fair buy if they cue with one shaft sells for $400 with no other shafts or case. If you want a bargain, talk him down a $100 or so. Or offer $500 for the cue + original shaft (you'd want that for value) and 1 Predator shaft if you already have a case. If you compete in tournaments and want a backup shaft if your tip flies off or something, add another $100 or 120. I'm guessing you already have a case.
If you start with "I already have a case and don't need so many shafts" he may start to lower the price himself unless he has others interested in the thing.