Some companies allow you to mark items sold that you do not want to carry around the sale with you or are to large to carry. Other companies only allow their workers to mark an item sold. You should know the policy of the company holding the sale.
Once an item is marked sold you are obligated to purchase the item because you have effectively removed it from the sale. It is not fair to the company and others to hold an item by marking it sold and then changing your mind. Someone else may have been ready to purchase the item and then left because it was marked sold. Also removing a sold sticker is not fair to the person who was first to decide to purchase the item. This all goes back to Estate Sale Etiquette.