Users will need to be authenticated (registered and logged in) prior to making a purchase. Sounds like you're on the right track. As a rule for the paypal integration to work, you should customize the login page information to prompt new users to register and then purchase.
Enrolment
Moodle and PayPal
Dán digaštallan lea giddejuvvon, it don sáhte šat vástidit dasa.