Umm, I think a reality check is called for here. eBay, Inc. in a publicly traded corporation on stock market. eBay, Inc. has 15,500 employees. Believe me, you are NOT going to be developing a site anywhere as complex as ebay.
So, rather than refer to ebay, I suggest you describe what you want to accomplish. Are you actually thinking of running an auction site, or do you just want a shopping cart feature integrated into your website so that you can sell goods or services online? If it's the latter, I agree with michaewlewis.