I used to specialise in e-commerce sites, and we would never require confirmation of an email to register and buy. The email address was part of contact details, nothing else, and so not critical to the process of buying something.
And as giraff says, lazy registration is the route forward - you should not have to "sign up" for anything just to buy. You should have the option if you buy regularly, and want to monitor your purchases, but that is a separate requirement.
The only time an email address should require to be validated is when the email address is the core information you are gathering - for example, to send electronic versions of products, or for newsletter.
You should, however, send a confirmation email, so that if I were to put in your email address, you would know, and get it cancelled.
But my perspective, as a user, is that the least I need to do to get something from you the better. And waiting for an email may just be the thing that sends me to a competitor.