If you receive $18000 from ebay, of which $9000 is profit, you would not report the $9000 as income?
There are a lot of non-business-savvy types who read this thread and rely upon it for advice. All of you should note that the advice from Acme above is tax evasion (aka tax fraud), which is a criminal offense in the US. That is a gamble that I would advise you not to take.
This stuff is not rocket science. Take a look at the 1040 Schedule C to get an idea of how it works. This is a schedule which attaches to your form 1040 (aka tax return). The schedule C is for a sole proprietorship, which is the business entity which you have by default chosen if you have not formed another entity (corporation, LLC).