Shop doesn't update after completed exchanges without cycling through the shopchests again

As identified by Yodabird19:

> would it make sense to have the mod read messages like `Successful exchange!` or `Shop does not have enough in stock.` (perhaps double-checking via measuring changes to the player's inventory contents) to infer changes in a shop's stock without relying on a player tabbing through the exchange list menu explicitly?

> I noticed when I bought out fairhill's prismarine shop that tradex still listed the shop as having stock, since I didn't tab through the list a second time after completing my purchases