try ebay, my friend bought a list price, $300 phone, for like 120 on ebay. he called up his providor (verizon) and gave the SSN number and it was activated in like 10min. the only thing you have to worry about is if the phone is stolen and reported as stolen, then if you try to activate it, you'll have some problems, since the SSN will be flagged. Then you will just have to give the phone company the ebay sellers info and let them take care of it.
But I have not heard any bad stories about buying phones off ebay yet.