But why don't u buy new one instead of a second hand phone however its price seems much reasonable and person selling this phone wont sell you much less than what it originally costs him as this one has got recently launched so if you opt for it in comparative less price by a second hand phone its not at all good idea.
And i would like to mention (coz i bought it on contract jst 2 weeks ago with orange) if the phone is locked to orange (if bought on contract by the original buyer then it wont be possible) or any other network provider then it wont be beneficial at to you at all coz orange (business monopolies) have built it specially with a built in "locked to orange" android version (currently v2.1) that restricts contract users to upgrade the OS version to latest version available while all other smart phones running android version could be easily upgraded to latest os version as per an individuals will however this wont be possible on orange san francisco on deal with orange. So in my opinion it will be much better to buy it sim free rather being confined to them.