Well...I bought mine from BeachBody but plan on selling it soon on eBay. I have no idea what is on there now but given that mine is definitely legit I'm sure others are too.
I would check feedback and sales history. I'm sure I could be wrong, but someone having multiple copies of P90X would definitely seem odd to me. I know there are people who are authorized etailers of Cathe's products that sell on eBay but I don't know if that is true for BeachBody stuff.