They would be worth what the market decides. The only way to know the figure if for someone to try and sell one at, say £500. See if someone buys it.
I know where you are coming from, and it is interesting,
I paid £500 for a prev model neox212t. That was once a £800 cab too, but I'd not expect to get more than about £300 for it. I base that figure on my usual calculation of 2/3 of the shop price at the time. This usually seems to be about right. So, valuing yours at £300 I am actually being generous in my calculation given what you paid for it.
Ethically, if someone tries to sell one at a price which is too high, no one will buy it.
Personally if I had just bought one of these at £380 and wanted to sell it now, I'd want £300.