50% of new if outside warranty always seemed to me to be a reasonable asking point when I’m selling and that’s how I bid when buying.
How did I come up with that number?
I bought a HK USP40 pistol about 25 years ago. Hated it. Ergonomics, size, fit, .40 cal impulse/noise etc. (my usual carry gun is a SIG P229 in 357 SIG, FWIW).
As the saying goes, it seemed like a good idea at the time.
Pistol sat in the safe for a few months, I took it to the LGS (Local Gun Store) where I was offered 50% of new price. Oh. I then sold it through Gunbroker to a guy about 20 miles away for 70% of the new price and he thought he was getting a great deal.
I consider what I “lose” in a sale to be “rent.”
So I bought the gun, ran a couple hundred rounds through it, took it to the range a couple times - what I “lost” might be close to what it would’ve cost to rent the gun, the lane, and the ammo from the local indoor range over that time so it’s kind of a breakeven point and a “stupid tax.”
Stereo gear? I see guys selling cartridges that were $6,000 new with - allegedly - very low hours for about half that but, there are many items on, say, USAudiomart that have been there for a year or more and the asking price is so close to new, forget about it. They’ll box the things up, stick ‘em in their attic and sell in ten years for more as “vintage.” Maybe.
This is called “Laissez-faire economics.” A thing is worth what someone is willing to pay for it, no more, no less.
It’s why I can’t watch another episode of - even a commercial for - “American Pickers” where you see some guy who’s a couple heartbeats short of a pine box and a wake telling the boys “I know what I got and it’s worth a lot more than that” but who’s buying? Iffen it don’t sell, it’s worth nothing, and it’s going to continue to sit in your garage collecting dust, rust and rats’ nests and excrement. That’s how I once bought a 1937 Harley Davidson knucklehead (EL?) for $200 back in 1974.
JMHO FWIW YMMV