First of all, let me apologize for non-functional version here.
Secondly, this version allows you to add custom items to shop - keys, or any custom item you want in the way you would add normal items.
Before, this would not be possible due to some limitations of skript, but I have decided to make a "workaround". Indirectly.
This is because skript does not compare item nbt's for some reason, and thus, if you have two diamonds named differently, and sell them for another price, you will still pay for the first one.

Due to this, I have implemented this workaround, which allows you to sell literally anything.
Next update on our server will be pushing commands to shop.
I'm sure this will not take any more than few tens of minutes, but I will push the update after it's been TESTED.

Thank you!