In my industry the old saying was “your first loss is your best loss”. Meaning that insulting offer you turned down won’t look so bad 8 months from now when you are still sitting with unused equipment while it ages. Which really bothers some owners because their rationale is “just listen to it, it sounds great!”. And by extension, it MUST be worth more.
From a different angle, if I buy a speaker new, and 5 months later the manufacturer introduces an update version, my older version drops in perceived value even more than just depreciation. But aside from manufacturers of real quality (Bryston 20 year unconditional warranty), other manufacturers HAVE to keep introducing upgrades or new models to generate interest and sales.
But the key to it all is the market size. If more new people come in at different price levels (depending on their disposable $) then they feed off the new and used market. So they generate purchases as they climb for better equipment and generate sales of used stuff when they tire of it. So long as there is a buying pool, there is enough demand to keep the resale values in order.
Which is the scary reality I suspect is going on...I have cases of movies on VCR tapes. I can lower my price to zero and it won’t sell.