Hi,
When I click on certain NPCs connected to the Shop plugin to make a purchase, I get the following error message in the console:

(14:35:11) | Failed to call hook 'OnEnterNPC' on plugin 'HumanNPC v0.6.7' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.HumanNPC.OnEnterNPC (BasePlayer npc, BasePlayer player) [0x0001e] in <e191a0640c9b4e7f8481f855e1917453>:0

  at Oxide.Plugins.HumanNPC.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00d0d] in <e191a0640c9b4e7f8481f855e1917453>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <42f9bedc659b4f4786eb778d3cd58968>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000de] in <15f61ddda771464d8246ebdce8ff4811>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <15f61ddda771464d8246ebdce8ff4811>:0

But the Trader itself works fine it's just the error message in the console.

Does anyone have any idea what this might be?

looks like you have

OnEnterNPC enabled on the npc check the datafile for the npc.