Correct, I'm using the default shop. Thank you so much for the script, it works great!
A couple notes though. If I try to go to the sell menu and have nothing, it goes to the item select window with a blank space highlighted. This seems a bit clunky, I think denying the player access when there's nothing to sell would be smoother.
Also, after backing out of the Sell mode item select window, when it returns to the Buy, Sell, Cancel window, everything is grayed out as if it's still behind the item select window. I can interact and close the window and everything just fine, it's just the visual of a darkened window is still present.