I will try to explain some things, sadly I had to rush the game for the jam
Materials are just farmable yet, sadly, I didn't had time to implement use for them till the last day of the jam
You can trade with NPCs, if you get near there is a balloon with "T" on it, at the game page and help file mention the command, pressing T will open a trade window kind of like Skyrim, every NPC has their own inventory with their own Gold :D
The Stage Level means the level of the monsters inside of it
I agree that I could remove some of the mechanisms that are not implemented yet, but as I want to make a full game out of it, I did let them inside, after the game jam I will probably drop a new version with a lot of stuff, probably will be a demo of the full game by then!