Despite not being a Hero, Saint, or even a Demon King, I was summoned

I was summoned to another world. Not as a Hero, not as a Saint, not even as a Demon King, but as a regular contract demon. You know, like the ones that come when you sacrifice a chicken or something on top of a magic circle. It's a bit weird, especially since I'm human, but I've just decided to go with the flow for now. Which apparently means becoming the employee of an item shop.

(What's that? You want to know what cheat powers I've been given? Well, how about an inexhaustible source of mana substitute? ...Holy crap, I have an inexhaustible source of mana substitute.)