You do not have to buy anything when you register. Registration is completely free and without any obligations..
You can buy without registering yourself or you can register to have additional benefits like the use of personalized product lists like MyList and Favourites or viewing the order status of a current order, viewing past orders and printing out invoices for past orders. Another benefit is that you do not have to re-enter your details with every order. You can even store several delivery addresses and choose the appropriate one with a click of the mouse when placing an order.
If you do not want to register, but want to buy products, just put the products in the basket and move to the checkout. At the checkout you can submit your order after you specify payment and delivery details for your order.