Have you met thief before ?

[hellatze]

I never met thief since we always adventure on small village and killing cultist.

What i must to do if i find one ? How to protect your item from them.

[The Sideromancer]

Any sane thief wouldn't go after somebody with the wealth of an entire city invested in weaponry.

As a DM, be prepared for the PCs to hunt the thief to the ends of the multiverse. In a way, this makes complete sense: a +3 sword cost more than a raise dead.

Having your stuff Arcane Marked you could Instant Summons it later as well as making resale difficult for a thief ("you expect me to believe you legitimately acquired a sword with this mercenary wizard's signature on it?")