Thank you so much! :D The key to buy/sell is ENTER. If you highlight the item you want to buy/sell, the key will appear on the top of the screen. :-)
I wasn't sure how to highlight/move between the items, if I hit enter it closes the shop.
Edit: Nevermind! It's working now, Idk what the issue was before. I restarted it a few times and it worked.
Oh cool, just saw your edit! Great, let me know if you have any other issues or feedback! :D
(Just to clarify: arrow keys are used to move around the shop, you can buy/sell with ENTER and exit the shop altogether with TAB. The game is keyboard-only so it sounds like you had a mouse bug, if it was used.)