Helter,
Considering everything you've mentioned, check out Zen Cart .. it's a bit of a beast to setup, however I believe there is a credit module available for free.
That being said - a user credit system is quite large, you would have to implement whatever method they use to acquire credit (credit card,etc) as well as work on how to remove credit, either ill-gotten, or from a return.
Also - because the credit amount will be stored in the database security will be a very large concern, and I can't recommend anything I've seen on the internet as it will not be secure out of the box.
I would suggest learning PHP & MySQL, learning mysql_real_escape_string() and learning how to implement something like htmlpurifier to cleanse any user-submitted content (including the form where they submit the number of credits they want to buy).
You will also want to write a module to verify the amount of income vs credits being given in case someone does find a loophole in the initial security of the site.
All that being said, if you do write something yourself, feel free to post code and people here will help you with security & efficiency.
