Hey there @metal_man, so Void items are a bit harder to get than they used to be -- in other words your guess was right, they're rare. Some players like to display them in their usershop, not intending for anyone to buy them, so they price them high. And you're mostly right that g0 items are useless -- some might say that g0 ingredients could help manipulate the grade of a rarity excluder, but I personally don't like to think that hard about my rarity excluders... xD

Anyway yeah, the value of Void items comes exclusively from their rarity/scarcity and nothing else. I personally wouldn't buy one, but if I found one I'm sure I'd put it on my usershop for the people who collect them.

Hope that answers your question :)