i tried out the last update of your Plugin.. (with the Spigot 1.13 pre7)

[08:54:21] [Server thread/INFO]: Cloude_Lecaw issued server command: /plugman load Shopkeepers
[08:54:21] [Server thread/INFO]: [Shopkeepers] Enabling Shopkeepers v2.2.1
[08:54:21] [Server thread/ERROR]: [Shopkeepers] Potentially incompatible server version: v1_13_R1
[08:54:21] [Server thread/ERROR]: [Shopkeepers] Shopkeepers is trying to run in 'compatibility mode'.
[08:54:21] [Server thread/INFO]: [Shopkeepers] Check for updates at: http://dev.bukkit.org/server-mods/shopkeepers/
[08:54:22] [Server thread/ERROR]: [Shopkeepers] Incompatible server version: Shopkeepers cannot be enabled.

``

I uploaded an experimental update: Try v2.3.0
I am rather buisy currently so I might not be able to fix any upcoming issues until around 18.08.2018.

The 2.3 works for me :D