I wanted to add some bartering items. Sadly, none of these items were shown in the bartering category. I tried to obtain these items and it worked. Just not shown in JEPB.

Hello,

how is it going with this feature ? :)

I think this happens because you need to take the loot table entries from client world loot manager and the datapack only changes the loot table on server which isn't synced to client.
I think this can be fixed by sending a packet with the changed loot table to the client.

I'll take a look as soon as I can, thanks for reporting!