If I try to buy an item from a main shop, but don't have enough sP on-hand to buy it, the item is still removed from the store. I don't get the item in my inventory, but the shop still acts as if I'd bought it and its stock of that item goes down by one. I did a couple tests to make sure it wasn't just other people buying the item in the meantime, so I think I can safely say this is an issue with code.