I would add to the above list:
Keep good records
Income (profit) from an eBay business (side-business or otherwise) is taxable, but its not necessarily simple. For instance, to determine profit you have to keep up with costs. Expenses such as eBay fees, etc. are deductible. And so on...
Of course, the argument can be made that most people don't do this properly, however, that doesn't mean it shouldn't be done. Also, the IRS has specifically stated that they are going to target eBay selling in the future because they know it is going unreported by many.
(Note: If you sell an few personal items through eBay, that likely doesn't constitute a business. You probably don't make a profit and you don't meet the other marks of a 'business' so don't worry. However, if you are buying and selling, making a profit, etc., then you have a business. Fortunately, operating as a business has some benefits.)
Here are some resources:
www.ezinearticles.com
TurboTax: Tax Center for eBay...
MSNBC: Taxes... eBay