This will probably produce a single-reply answer, what does it mean if a merchant has like -5 of an item? something about the items coming back into his inventory when bought out or something?

That means that after 24 hours, he will reproduce the items. But if there was 5 of them, and you steal 2, there will be only 3 infinite items. If you steal all of them, they won't reproduce anymore.

Actually I'm pretty sure they reproduce whenever you close then open the bartering window again. It takes 24 hours for the money to restock but not infinite items...

My mistake. I just tried it in Morrowind, it seems that you are right, Switch.