"Plumber's Trumpet": {
      "buy": "200",
      "img": "https://rustlabs.com/img/items180/fun.trumpet.png",
      "item": "plumber's trumpet",
      "sell": "1"
    },​

Try this instead.

"Plumber's Trumpet": {
      "buy": "200",
      "img": "https://rustlabs.com/img/items180/fun.trumpet.png",
      "item": "fun.trumpet",
      "sell": "1"
    },

"Plumber s Trumpet": {
      "buy": "200",
      "img": "https://rustlabs.com/img/items180/fun.trumpet.png",
      "item": "fun.trumpet",
      "sell": "1"
    },

Default

Try this instead.

"Plumber's Trumpet": {
      "buy": "200",
      "img": "https://rustlabs.com/img/items180/fun.trumpet.png",
      "item": "fun.trumpet",
      "sell": "1"
    },

I tried this but didn't work.  I have never had luck when using the short name in this config.  

Failed to call hook 'ccmdShopBuy' on plugin 'GUIShop v1.5.9' (FormatException: Input string was not in a correct format.)
  at System.Number.StringToNumber (System.String str, System.Globalization.NumberStyles options, System.Number+NumberBuffer& number, System.Globalization.NumberFormatInfo info, System.Boolean parseDecimal) [0x00057] in <437ba245d8404784b9fbab9b439ac908>:0
  at System.Number.ParseInt32 (System.String s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x00013] in <437ba245d8404784b9fbab9b439ac908>:0
  at System.Int32.Parse (System.String s, System.IFormatProvider provider) [0x00008] in <437ba245d8404784b9fbab9b439ac908>:0
  at System.Convert.ToInt32 (System.String value) [0x0000b] in <437ba245d8404784b9fbab9b439ac908>:0
  at Oxide.Plugins.GUIShop.ccmdShopBuy (ConsoleSystem+Arg arg) [0x000d1] in <42d0d59d53394ada8e2eaec3309ea79f>:0
  at Oxide.Plugins.GUIShop.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00cd0] in <42d0d59d53394ada8e2eaec3309ea79f>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in :0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in :0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in :0

If you just take out the ' in the name it should work without error. + Do the same thing in the buy and sell sections of where you put the item in.  

Currently, the JSON Format does not support names with certain special characters... 

"Plumbers Trumpet": {
      "buy": "200",
      "img": "link",
      "item": "fun.trumpet",
      "sell": "1"
    },

 Failed to call hook 'ccmdShopBuy' on plugin 'GUIShop v1.7.0' (FormatException: Input string was not in a correct format.)
  at System.Number.StringToNumber (System.String str, System.Globalization.NumberStyles options, System.Number+NumberBuffer& number, System.Globalization.NumberFormatInfo info, System.Boolean parseDecimal) [0x00057] in <fb001eTtTPKGxHjPd6F7BVwCFgxuUX711GLG6>:0 
  at System.Number.ParseInt32 (System.String s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x00013] in <fb001eTtTPKGxHjPd6F7BVwCFgxuUX711GLG6>:0 
  at System.Int32.Parse (System.String s, System.IFormatProvider provider) [0x00008] in <fb001eTtTPKGxHjPd6F7BVwCFgxuUX711GLG6>:0 
  at System.Convert.ToInt32 (System.String value) [0x0000b] in <fb001eTtTPKGxHjPd6F7BVwCFgxuUX711GLG6>:0 
  at Oxide.Plugins.GUIShop.ccmdShopBuy (ConsoleSystem+Arg arg) [0x000d0] in <926db41127c640d8b4db6d29fbfb8b74>:0 
  at Oxide.Plugins.GUIShop.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x011e1] in <926db41127c640d8b4db6d29fbfb8b74>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d09a1f46ca2f4432811bcfe45ad13c7b>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <cf88a28c7fb44d36890d85a78331cc9d>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <cf88a28c7fb44d36890d85a78331cc9d>:0