nope...you are screwed...i feel for you...you may be able to get a rep that feels sorry for you but i doubt it...we are actually supposed to confiscate stolen phones and try to get them back to their owners
ask for the ESN upfront first, double check the esn on the phone matches what he told you...and call into sprint before any money is exchanged to make sure that phone is free and clear
these are the dangers of buying phones off the net