Why cant the cat just tell him? Also, Theo shouldn't be this worried about the MC. Until now, the MC never tried to even touch him and since there's no conditions forcing Theo to sell the things, he can just don sell a thing to cancel the contract. Back to the question: why can't Theo simply tell him "I'm a novice merchant and novice merchants got their items creators name censored in the status of the item when it's sold. It's not me, it's the system, so you'll got to wait until your name start to show up in the items"
There's absolutely no reason for Theo to not share this information.