SSL certificates encrypt the conversation between browser and web server (e.g. Apache). Moodle does not know (though I dare say it can find out) if it is running on over http: or https:. If you are using Paypal then they will be using their own SSL certificate to protect that conversation. If you are not collecting sensitive data you do not need an SSL connection.
Having said that you might choose to keep the certificate going as it may help project a professional image of your service to your customers. It shows you take confidentiality seriously. It also helps prevent "man in the middle" attacks such as from the Firefox addon "Firesheep" (or whatever the latest variant is called) whereby snoopers can steal login credentials and impersonate students on your site. Why they would want to do this is beyond me .
Hope this helps,