If any commoner starts stealing from any King, any noble should make sure that commoner gets punished or more commoners will try this.
"should make sure" does not equate to
"will make sure". There are many, many players who would rather have their character laugh at the king for losing his shiny bauble to a commoner, than to have that commoner arrested, then have the judge sacrifice his H/P to take the item away, then give it to another commoner to have it delivered back to the enemy king.
Let's ask the question this way:
Have you ever successfully pursued and punished an advy for stealing a unique item from one of your characters? Note that your story
must include the advy being pursued to a non-allied realm, where the final resolution involved the advy's execution and/or recovery of the lost item.
How to get a trusted commoner? figure it out, there many ways.
A guild or a religion could be useful.
Really? Wow... who would have ever thought there were ways that you could find a trusted commoner? And using a guild or religion to find one? Damn! Why didn't I ever think of that?