When someone want to buy something from nexus, they will fist asked to create an account/log in. After that they enter their personal data in nexus (name, address, etc..). Here is the beauty Nexus allows you to set buyers to be upgraded into a new group when they buy a product and also if it is a subscription it down grade them.
What I did is as follow:
In my forums I renamed members to be Visitors, then adjusted their permissions and only allowed them to post in pre sale forum or test forums while the rest is blocked for them. When they buy my products they are automatically upgraded to a customer rank and gain full access to the forums. If they cancel their subscription or don't renew they get downgraded to ex-customer group which blocks them from most of the forums.
In regards to your question i think I gave you a full answer :).
The best thing about IPB is the adminCP, almost perfect control over your site.