Actually, it's not reasonable to assume that everyone "entering the checkout-process" is a real buyer.
I've been known to not only enter, but even complete registration or checkout on obvious spammer-sites and phisher-sites, filling their forms with complete garbage.
So even -completing- the checkout-process is likely to overcount the number of real customers. At the very least, they should check if the person is willing to give them a real credit-card-number. (yeah, I know, there's ethical dilemmas in that, even if they don't actually charge anything)