I have always looked at buying used gear for half of what it sells for new. Then, depending on how much I really want it I may go up a little more. At some point there comes the threshold that if I am paying close to 75% of its new value, I would prefer to just get a new one. When I sell things, I will typically aim for about 65-70% of the new value and plan to negotiate from there. Pending condition of gear, price could be lower... Good luck.
“He doesn't know the meaning of the word fear, but then again he doesn't know the meaning of most words”
- Bobby Bowden