IP address isn't the issue. To the world your router only has one visible IP address even if you have multiple computers with individual addresses connected to that router. While your browser caches your Etsy logins (or any login for that matter) so subsequent pages opened remain logged in to your account, simply opening an incognito page lets you act with your site like a non-logged in user. Similarly, different people can use different computers in the same house at the same time to access Etsy, and only the browser session logged in will prevent 'buying your own items'.
I routinely - on the same computer I am logged in to Etsy with - open a different browser and without logging in there, add items from my site to a cart. It's the only way to check if checkout is functioning, or what Etsy is displaying when your customer hits the cart button. I really can't see the need for Etsy stopping add to cart for shop owners.
A smart site would realize the items added belong to the owner, allow the ADD but prevent checkout, and empty those items when the browser session ended.
