I have no Idea how the merchant work, but this is a really cute game. I wish there were more polishes though as one of the most important aspect of how a game is fun is determine by its JUICY NESS or POLISH. The level design is actually pretty cool with ramping difficulty mechanics, I like the creeper skeleton guy. And this game would definitely be x999 more fun with polish like effects when hitting and sound effects. I also couldn't tell how much hp the skeleton has left, especially on the big guy(I thought he was a boss). So I would recommend adding sprite for damaged skeleton or health bar.
Great jobs!