You can't escape in the middle of the animations. You can only go forward by pressing "jump" button.
If your key binding is default, during shop menu, the OK button is "Z" in keyboard, while that is "Skill 2" in Xbox or PS controllers("B" or "O").
And the cancel button for exiting the shop is "Jump" for either keyboard("Space") or controllers("A" or "X").
I can buy and sell items normally in the game.
(There is another bug that you can sell items those you are equipping although.)
Maybe the point of this game is to put many things into the tummy, grow the things(tentacles' sperm -> meat pieces, egg -> slug) inside, enlarge the tummy, and continuously birth all the things inside.