Stuff depreciates, but money does too. And there are so many other factors - vintage value, availability, modding etc.
You might have bought a P Bass in 1962 for £79 and thruppence-tuppence h'apenny but that doesn't mean selling it for less. A MIM Jazz bought last year probably won't go for more. Some new Jaydee custom basses have an 18 month waiting list, so they probably will go for more. Other stuff can be improved (or indeed wrecked) by modding.
I guess you win some, you lose some. I once bought a Fender Rhodes eleccy piano for £100, and sold it a year later for £200. I also bought a Trace Elliot Series 6 full stack a few years back for £600, and sold it for a disappointing £250 on fleaBay. I subsequently bought an American Ampeg SVT half stack for £450 from the same place, and shifted it for more when I sold it recently.
I guess it's all down to market forces. We've all known on BC how certain brands can fall in and out of love...
BB