Are these old, second-hand soundcards really valuable? In that case: why?
I was truly sad to lose 16-bit ISA slots on motherboards because it meant I no longer could use my GUS. Held out as long as I could, disabling the audio built-in to the motherboards at the time.
By the PCI days the cheap cards were better quality; the market differentiated on names and flashy features.
I recorded using SB Awe64 Gold cards on ISA machines, and Ensoniq es1371 cards on PCI machines.
It is pretty unique for time period. Basically best you can get.
And there is not many of them compared to let's say Sound Blasters.
And other products are more interchangeable. Like it matters less what video card, network card and even which Sound Blaster you get. So there is lot more options among those.
You can change the eBay search criteria to show either previous listings or previous listings that sold.