[BUG] Shops do not update when multiple players are observing them

[src52]

Describe the bug
When two players open a shop, the shop interface does not properly update when a purchase or sale is made to the shop. The other player is not made aware of it, and can still buy the amount of items that they see in the shop, even if it doesn't reflect the actual amount available to purchase.

To Reproduce
Steps to reproduce the behavior:

1. Log into two accounts
2. Open the same shop
3. Buy or sell an item

Expected behavior
Both observers should instantly see the changes made to the Subject ItemContainer.