It's the ovary sac and oviducts of an octopus (of some description), but not from (above) Tremoctopus (the distal oviduct of this genus is exceedingly long and thin, and the eggs are minute - unlike the large eggs in the sac of the ovary for sale).
Why would anyone place the ovary & oviducts of an octopus on ebay? Rare? My word! What nonsense!
That's an isopod; looks liike a stomach content specimen though. Rare? Not! It's the kind of thing anyone from a museum would throw in the bin as it is completely worthless.
I want to call it Seriolis.... my memory is failing me; perhaps Acutiseriolis .... but can't find anything; go to the bottom of this link you'll see the live thing.
There's just nothing (almost) online about this beast, which makes me think I've got the name wrong (they are so common at ~ 1000m around New Zealand). I know there was a lass in Australia working on the group, and that she was changing all of the names around.