Thanks for the quick response, I wasn't able to get back to look at this until the weekend.
No limits on the shop. Player has potions at the start of the game and I have also tried buying items from the shop and I cannot sell them back.
I enabled the option 'Sell Without Items' and found that with that enabled, I am able to open the shop UI and can sell the potions I have. With 'Auto Close Sell' the menu closes after every potion I sell, even if I have others. It's like it's not registering that I have things to sell... But I do, and I can sell them if I set it so the menu opens regardless.
I'm not sure what would cause this, especially since it's working as expected on your end.