You can duplicate items exploiting how trades are locked after an item runs out.

Steps to reproduce:

1. get two players (or simulate two players like I did in the video below, by changing the owner of the block)
2. put at least two of any item in the shop inventory
3. create two listings for said item, for example one diamond for one coin, and two diamonds for one coin
4. get the other player to buy out both listings
5. profit

Mod list:
cloth-config-7.0.72-fabric.jar
fabric-api-0.57.0+1.19.jar
numismatic-overhaul-0.2.3+1.19.jar
owo-lib-0.7.3+1.19.jar

Fixed in commit 8a3f451
It will be available next release