You don't need to be a Zibbet member in order to purchase from a Zibbet seller.
You simply need to checkout as a guest.
How to checkout as a Guest
Once you've added all of the items to your cart that you wish to purchase, click on the "Proceed to checkout" button.
You'll see this "sign in" modal appear. Simply click "Continue as a guest" at the bottom of that screen. And, that's it!
You'll receive the sellers email address in your purchase confirmation email from Zibbet, so if you have any questions you can just email them directly.