If you're buying a headset, check with manufacturer website to ensure you're not buying a counterfeit product.  There are many, many counterfeit products on ebay.  For example, Motorola does not make pink headsets.  So if the ones on ebay are counterfeit products.

As to manufacturer websites, see for example http://www.motorola.com/original where you would learn how to spot a fake Motorola headset.

Good luck and remember, if it's too good to be true, it probably is.