check the folder that contains all the javascript files, it's from them that you can edit the whole thing done as well adding some features that doesn't exist from the rpg maker mv software. here a screenshot as example:
i hope it helped you out.
ps: your game was fun to play!
edit: you can edit the code source from the mjshi's plugin, you will be able to find the "sell" function that you could remove completely. that is where you could find the information as well the code to edit: Remove the "Sell" option from the shop menu completely? | RPG Maker Forums (rpgmakerweb.com)