Why can I add my items out of my store to my cart?
You can't add items to your own cart. Only potential buyers can.
You can't buy from yourself.
Why would you want to do that?
Lots of new sellers want to buy one of their own items to see how the whole process works.
Etsy has no way to preview how a sale will look and what happens. New sellers fall for the we need your email to complete the sale, because they have no idea how a sale is supposed to work.