This mod has a bug that, when purchasing items that differ based on NBT Tags, squishes existing item stacks together. I'm currently working on a modpack that is (hopefully) going to update this weekend.

The bug occurs at Line 263 of VillagerMarketContainer.java

It can be fixed by changing the line to:
else if (!inv.isEmpty() && inv.isItemEqual(result) && inv.areItemStacksEqual(inv, result)) {