First of all this is fantastic idea that I would love to see further developed into a full game. The shop where everything is free is a brilliant interpretation of the theme and would make a great mechanic for a rouge-like. Sound effect and other forms of feedback would help the game a lot. There are many elements you could add such as a limited ammo of rocks that can adjusted through the shop. Also being able to see your current stats would help players make more informed decisions. You have an amazing core idea here, and I highly recommend you developing further into a full-fledged game.