You have to make sure users entered the correct email address prior to proceeding with the order, otherwise billing information could reach the wrong person or be lost.
Also, just because you need an email address doesn't mean users have to sign up: Should registration be optional on an e-commerce site?
If the users need to register prior to ordering, then the email confirmation should be a mandatory part of the registration, without which the account is not enabled and log-in is not possible (with perhaps the exception of logging in in order to correct the email address and resending a confirmation request).
I personally get signed up to sites I've never heard of (with my shorter email addresses), every few months and the processes of attempting to remove the accounts or unsubscribe is very annoying. Besides the hassle, sometimes I end up getting personal information about people I don't know.
In order to prevent this all you need to do is ask the user the users to confirm their emails before activating their accounts.
Edit 2 to address comment regarding UX (to other answer):
Entering the wrong email address (either by mistake or intentionally) is a worse user experience than validation. If the user needs an email to sign up, you might as well validate it.
If the user preforms more than one purchase in the site, you do not want to revalidate the email, so why use a different UX for the first purchase than the others. It also may annoy the users, since they are already signed up, yet to order something they need to do something that isn't related to the actual purchasing.