No, _not_ all sites are safe, even if they claim to be. Some of them are intentionally trying to abduct iPhone information. Actually, one member last year listed a site that they ‘claimed’ that you could put your serial number into for verification, and that was later found out to be a hacked site, and the thread was deleted.
Swappa is safe. Or alternatively, you can contact your carrier provide them the serial/IMEI number, and they can give you feedback on the phone.