I really like the idea of bending the tropes of the buying from a store.
The art is really good, and I hope you save it for future use.
I think the core concept has some legs, closing it in a core game loop would make this a lot of fun.
Maybe some feedback on the fact that something happened would help with the guidance. I didn't realize I had used the door to steal his key until the second time I played it. The feedback could be as simple as a specific noise, particle effect, etc.
Overall, a great entry :D