I'm amazed at how much people will pay for used equipment, sometimes paying more than it would cost new from a reputable retailer with free shipping and a liberal return policy.
I wonder if they don't know about google.
My target price for used is equipment (both buying and selling) is 50% to 70% of what I'd have to pay to buy new. More than that and I'd rather have new. And what the seller paid is irrelevant to me. If I can buy something new for $400, the fact that seller paid $450 for it new isn't going to get me to pay the seller $390 for it used.
The exception is something I want that is no longer made, then there is no retail new price to compare. What it cost 5 years ago is irrelevant.