Hi! Thank you for trying the game. 
To answer your points:
- This is intentional, although I should perhaps have provided some indication to this effect: it’s built with certain rogue-like principles, including the fact that you may simply not have the reosurces to buy items when they’re presented. (You can see this at times in the shops of games like The Binding of Isaac and Our Darker Purpose, I believe.)
Note that items that you don’t have sufficient potions to buy are shown with their cost and backdrop in red to indicate that they are unavailable.
- This is my fault, I fear: there is a way to move forward, but it’s perhaps not very clearly labelled as such. In the shop screen, once you’re ready to move to the next level, look for the door in the bottom-right of the screen. Click on that door and you should enter the next level.
There is one significant bug here, however: if the window isn’t at the intended size (if, for instance, the issue metioned in my last post above affects you) then the image of the door may be off-screen; in this case, just click towards the bottom-right of the screen and you should progress.
I’m honestly not sure of how best to fix this last: the window-size issue doesn’t seem to be an issue in my own code, based on my thread regarding the issue. On the other hand, the shop uses a single large image for its backdrop, making alignment of the image and the (invisible) door-button problematic. (And if I were to accurately locate the button with the door the former might end up as much off-screen as the latter in the case of the afrementioned bug.) 