This discussion thread is primarily for a module/tool that is called the Workshop. I think that you are primarily interested in how to control enrollment into classes.
The newer versions of Moodle allow you to use SSL/shttp and have a variety of enrollment options. I think that Paypal is available as an enrollment option since Moodle 1.6. Your best bet is to go to the Enrollment Plugins discussion and read what has already been posted.
Here is a list of the steps you would need to take for Moodle version 1.7 (mostly from memory of setting up the authorize.net enrollment option <-- so some may be wrong)
1) get the certificate for your cite so that SSL will work
2) verify SSl by trying to access a file via https
3) turn on https by going to Administration -> security -> HTTP security and checking the box associated with "use https for logins" (If you can not login at this point you had better hope that you have access to the moodle database of your site so that you can reset that checkmark! Information about how to do this IS available in the discussion forums here at moodle.org)
4) Go to Administration ->Courses -> Enrolments (<-yes, for some reason Moodlers spell it with just one "l" must be a Southern Hemisphere thing!?!) and then select the enrollment options you want to use and configure them.
5) I haven't used the PayPal enrollment option but I am assuming it is similar to the authorize.net enrollment. If so, go to the actual virtual classroom (or create one and then input the payment information in the settings for the course.)
At the end of this list I realize that I do not know if you need SSL/https for paypal - if not then many of the above steps will be un-necessary. Moodle allows you to use multiple enrollment options and so if you want to have some people pay with PayPal, other with VISA/MC/AmEX,etc, and others via passcode that they enter --> you can do it.
Using the WorkShop as a Class/Workshop Registration Module
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