Wirelessly posted (HTC Diamond: Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.11) Sprint MP6950SP)
Well, I mean a phone that can be reactivated. Some phones that are for sale have been reported lost or stolen and thus can't be reactivated legally. Usually, reputable sellers will state clearly that the esn is clean or bad. Bad esn phones can only legally be used for parts.
Look for one that is branded for your carrier to make things easiest.