Stick with PHP since you are already comfortable with it.
I've tried using Drupal but it's too damn complicated to get going. CodeIgniter is still based on PHP4. If you think that's a good idea, I guess CI is an option. There's also Kohana which started as a PHP5 fork of CI. I've used it a little bit and while I don't love it, it's not awful either.
Wordpress has plenty of flaws but it's probably the best option if you want to get your store up and running soon. There is a plugin for anything you might want to do. Specifically there is the WP eCommerce plugin which has grown a lot and there's plenty of people to provide paid support when needed.
Link: http://getshopped.org/
Besides making a completely WP-based store, you can run it on Magento, osCommerce, hosted solutions, etc and you can bet there's a plugin or a guide to help you integrate those with WP.
So, should you use a framework or a CMS? It comes down to whether you want to implement everything yourself, or use an already proven eCommerce platform.
If no platforms fulfill your requirements, it's a choice between hacking other people's code or building it mostly from scratch. Sometimes it's just as hard to do the latter as it is to figure out how someone else's tangled spaghetti code works.
In conclusion, go RTFM and see if there's something that will do the job out of the box.