Erm.... I think you've gone way too far way too quickly. There are so many alarm bells ringing right now based on what you told me...

Firstly
Have you done anything like this before? What do you know?

Database
Which technologies do you have available? Access? MySQL? MSSQL?

Uploads
Which technologies will you have available? ASP? PHP?
How are you going to manage/store/organize these files?
How are you going to restrict file sizes?
How are you going to protect these files?

Shopping cart
Have you used Paypal?
Do you have a merchant account?
Have you ever built an online payment system?
Are you going to store sensitive/financial information? If so, HOW?

Lastly
What do you expect from this project? What gap are you trying to fill?
What will set you aside form YouTube, iLike and the other big guns?

I don't mean to discourage you from the idea, but I got the feeling you don't really know what you're getting yourself into and all the above are the most basic theoretical question you need to answer before you can begin to consider actually building this...

...if that's the case, may I suggest you start with something simple like a blog and familiarise yourself with web-design first before you attempt to make a living out of it.