Spirit shards are used as a currency as they can always be bought and sold for 25gp each, from pet shops. If you were to buy a purple partyhat for ~6.3b, you would buy packs of shards and unbox them all until you have 252m shards, then you would put an ad on the forums to discuss a price in-game. It's extremely rare for a partyhat to be bought on the G.E for max cash.
EDIT: I used 6.3b just as an example, i'm not sure what the exact price is. It's always advised to do your own research to get a decent price.