There’s a ghostly aura infusion?
From the raids rewards blog:
Unique Boss Rewards
Raid bosses also have some additional unique, thematic rewards. These may include miniatures of the boss or their minions—or even unique cosmetic rewards. One such unique reward that can be obtained in Spirit Vale is an infusion that, when equipped, will provide your character with a ghostly aura.
On topic:
the poly upgrades will stay tradeable, even if you use them.
So buy one now, make sure to slap it on a salvageable trinket and if you obtained the ghostly infusion, you can salvage the trinket and sell the poly. if you wish. Maybe you even make a little profit, if prices start to rise again.
And maybe the effects stack with the ghostly infusion, so might want to keep both.
I think the candy corn orichalcum amulet stays account bound, even after equipping it, so it might be a good choice to slap the infusion on it, as you can swap it between alts. And it has selectable stats.
Not 100% sure though, if it really stays account bound after use.