Howdy! By course registration key, I assume you mean "enrolment key" that is available for each course.
What you can do is create multiple groups within a course and assign different enrolment keys to each group. When you send the information to the purchaser, send them the key for a specific group you have created. Avoid sharing the enrolment key for the entire course.
I discuss this with a different purpose online at http://www.mguhlin.org/2009/06/moodle-tip-value-of-groups-in-moodle.html
Note that you can alter the group setup slightly, but the effect would still be that you could issue multiple enrollment/course registration keys for the same course.
Hoping this is helpful,
P.S. I may have completely misunderstood the intent of your inquiry, and if so, my apologies.