We have had a recurring issue with sign shops using barrels showing out of stock when used even though there is stock in the container.

For example, a player was trying to sell fireworks, but the sign is red and also reports that the shop is out of stock when you right-click on it.

This may be related to #83 since they are using named items, but we've had recurring issues with barrels + sign shops, usually remaking them with chests fixes the problem but we'd like our players to be able to use any container to make shops.

I cannot seem to reproduce this issue. I made a custom named firework with a couple of effects and even some lore and setup a shop to sell 1 from a barrel. After filling the shop with a stack I was able to buy one at a time until the shop was actually empty. I used Spigot 1.16.5 for this test with minimal plugins. I also did a restart to make sure it still worked after a restart. Perhaps you have a different setup? Please reply with your full setup as well as any steps that I can use to reproduce the issue consistently.

We are using paper 1.16.4 and quite a few other plugins. Essentials, Luckperms, shopkeepers, multiverse, world edit, world guard. Others but I don't know if they interact with signs or containers.

Do you think it would make any difference if this shop was created in the Nether versus the overworld?

You should test with minimal plugins. The nether should not matter. I will test with paper after work.