My personal rule of thumb is used = 30%!off retail right off the bat. No offense to certain folks but I've found that on enthusiast sites people tend to think their junk is golden. "Six months old, only used twice. Amazon is $900, mine is screamin deal at $750". Or even better "won in a raffle, still in box, never used. $800. Brand new, save $100!"
Even if it's still in the box, why would I give $800 to some unknown internet person when I can spend another $100 and get from a place I have some recourse if it goes south?
My long winded ranty way of saying you're probably better off going new in this case.