@daveyf
I suppose that depends on the unit. I'll agree that the used market is probably more price driven than the new market, a good value is a good value. I wouldn't consider a pair of speakers that retail for $12,000 to be a good value at $10,000 used, but a pair of speakers that retail for $20,000 for $10,000 used could be (if the age and condition are appropriate).
I'd wager that at minimum any used item should have at least a 25% discount from new retail price, and that's if it's in perfect condition with all original accessories such a buyer's remorse trade-in or dealer demo. Make it a few years old and it shouldn't be any more than 50% of new retail. Go to ten years old and it should drop even more.
Of course, really hard to find specialty products could buck that, and while I window-shop in the for sale section here, the asking price is higher than I think is fair for a lot of it. That's fine, people can ask whatever they want for something, and I'm not going to insult anyone with a lowball offer, I'll just keep my eyes open until the right deal surfaces for something that I want at a price I think is fair.
There's a pair of Revel Salon2s on sale here for $8,000, which is an amazing price, but the ad is rife with misspellings and typos, the photos aren't as illustrative as they could be, and there's something off about the tweeters - they're at minimum missing the twin wire guards they should have, not sure if they're even the original OEM tweeters because of that. If I lived in the area where I could go see them in person I'd certainly go do that, but that's too much risk for that much money for me from a distance.