My honest suggestion is to not reinvent the wheel. An ecommerce application is not something you can really develop in your spare time. There are too many components and areas of failure to keep anything really secure and worth using.
I would highly suggest you download and install Magento as an ecommerce platform (or osCommerce or ZenCart). Magento has a fairly good code base you can start with to create your own extensions and make it how you want. At least then you'd be selling items while you're developing in your free time. Plus, all the security and main issues with the cart, customers, and all that jazz is handled by a core team of developers who are paid to focus on that product, rather than devoting their free time to it.
Magento is written purely in PHP and utilizes MySQL. So yes, you can write your own ecommerce solution in PHP and MySQL; however, why do it when you can download something that already works and works fairly well 😉