I want to use my shopping cart system with eBay

Ok, this question gets asked quite a bit.

Basically the road map is the following:
- To interact with the eBay production servers, you would need a developer account:
http://developer.ebay.com

- Once registered, you would need to Self-Certify your application:
http://developer.ebay.com/DevZone/launch/SelfCertify.asp
They will send you your developer keys. It consists of three key sets.
They are like usernames.

- You would then go get your authentication token (Using the Keys they sent you):
http://developer.ebay.com/tokentool/
Note it will redirect you to eBay login, as a security measure. And then redirect back to developer.ebay.com page with your token. It is a long string of letters and numbers.

Once you have all that, free of charge, you will
then be able to interact with the actual eBay servers.
Self Certification will allow you to make 10,000 API calls per month, but unlimited Add Item calls.

What mostly matters are the fields in your product database table, since those will be the fields that will have to be passed along to ebay.
It mostly relies on formatting the product information, price, buy-now, title, description, etc, and sending it to eBay in a way they can understand it.