Auction house is a premium plugin that is featured on many servers. I am trying to figure out how to link the auction house to an npc. I run citizens as well, but I can't figure out how to tell the NPC that he is an auction house haha. Any ideas?
I'm not familiar with this plugin but I'm sure you can link the gui open command to npc (with an extra plugin Citizens CMD) But you are better off asking this question in spigot "plugin support" or in discussion section of this particular plugin.