I support my local dealer and then I know that all my stuff is authorized. An example of this is Focal products. They don't authorize internet dealers so when I see a set of Focal K2Ps on woofersetc for $600USD I just shake my head.
The first $1000 I spent on equipment was from Ebay, I would say without a doubt, DO NOT BY ON EBAY unless its an authorised dealer. Most likely it wont be.