I have a theory on what happens when a fall guy fall in the slime. The fall guy dissolves into nothing and only the costume is left, the costume goes into one of two places, 1: it goes into the shop, 2: if the item is not in the shop, it goes into a vault where it will be until the costume can be sold again. Don't worry, the costume is duplicated so you have the same costume by the time you go into the lobby.