Selling items rapidly and getting to the last item in your list can potentially lag the game and make it so you can mass sell whatever said final item is (since the sell button cant keep up with the lag and gray out properly). Trying to buy back any of the duplicates results in a critical error. I swear I've posted a bug like this before and it got fixed. Also not sure if the lag is because of mobile version of if it also affects computer versions as well.