They were very early high end cards; thats part of it. They had some cachet of exclusivity when new, because the SoundBlasters were what sold to people who wanted compatibility. GUS cards were known for lower noise.
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.
Multiple factors. Nostalgia even by people who did not own one. It was something to be desired.
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.
> If they get sold I do not know
You can change the eBay search criteria to show either previous listings or previous listings that sold.
Had a GUS Max with 256KB of memory that I upgraded to 512KB. When your other options were little more than beeps and boops when playing MIDI or spending far more for a SoundBlaster AWE32, the GUS was a relatively easy choice.
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.