Ways to approach creating a market website?

[shadowz]

This may be an awful, and probably not answerable question but its worth a shot. I have an advanced knowledge of HTML and CSS, but an EXTREMELY basic knowledge of PHP. I am trying to create a website where users have an account, and are able to sell virtual goods. I want to know what areas of PHP I should start learning in order to ultimately create a site like this. (example of site I am trying to create: https://opskins.com/ ) I'm not asking "How do I make this?" but rather, "Where is the best place to start learning PHP to the point where i can piece this together?"