This is less of a technical issue, more of a question. How do you use a buy sign with items that are longer than one sign line like experience_bottle.

Thanks

Possibly this fourms post on Spigot will help:
https://www.spigotmc.org/threads/need-help-with-essentials-shops-ids-issue.382723/

You can hold an item in your hand and run /itemdb to see a list of available shortnames you can use on signs. You can also add your own aliases for items by editing the items.json file.