it's better off getting a trusted company that meets label claims. There are several brands on eBay that are fine (i.e. Optimum Nutrition, Syntrax, etc)
the thing is can we really be sure just because it has a name brand like optium its not the exact same stuff as some of the generic 50lbs of whey you can buy on ebay. I mean some of these companies may just buy it wholesale from some other company that sells in bulk. However I may completely wrong.