Used anything, I generally try to be around 65% of retail, then may or may not negotiate from there.
Some things are worth more to any particular buyer or seller. There are things that are of "value" that may not be obvious in the sales price, e.g., can I drive to get it, look at it, put cash in someone’s hand, leave, and know 100% that I’m happy with what I bought and it’s arriving home safe and sound? That’s worth money to some people. Others may not want to fool with packing a heavy / bulky item and dealing with shipping. Maybe they price it to attract someone to come and get it.
What something is worth is what a buyer and seller agree it’s worth. Indignantly pointing at a blue book price and telling someone what their property is worth is rude. The price is the price, make your offer... if it doesn’t work out, move along. It seems to me that those who arrogantly act like "The Price Police" and point at a "blue book price" when buying, or even unsolicited when they aren't in the market, would also be inclined to get more than what the BB says when selling their own gear. Funny how that works...