I sell stuff on ebay from time to time. Am I required to report these sales on my taxes? If yes, why isn't ebay and other auction sites informing me about this like google in adsense?

These people don't really know what they are talking about. Ebay is an on-line auction site, and as such it can be argued that whatever someone is willing to pay on that site is considered fair market value for a product. As long as you are not selling the items to a related party, or a party of interest it can be considered fair market value, b/c Ebay represents a segment of the market, and what they are willing to pay for it is considered market value. You don't need to worry about selling things on Ebay, unless that is your primary source of income (i.e.- that is your job). In which case you would pay taxes on the difference between what you acquired the items for and what you sold the items for since these would represent your expenses and income respectively. Hopefully that helps.

–UPDATED TO RESPOND TO TAXMANNYC—

Even if he is a CPA w/ 10 yrs. experience he doesn't know what he's talking about. Perhaps CPA means Can't Pass Again. Here is an article that outlines what I initially wrote in my correct response to your question above. I hope this clears up the confusion.

http://www.wired.com/news/ebiz/0,1272,67035,00.html?tw=wn_tophead_7