When we were running our off-eBay e-commerce website, we had it set up in such a way that it was up to you if you wanted to create an account before you made a purchase. In other words, you could still make a purchase without creating an account.
You have to be mindful of people that don`t want to create an account just to buy a product from you. It is a big turnoff to some people if they have to create an account just to buy the widget you are selling.
Make your e-commerce website as user friendly as possible so that you attract as many customers as possible.
If a site requires me to create an account to buy, unless it`s somewhere I shop frequently at (Amazon, Target, eBay, etc) ... I won`t buy from them.