Since it was a used phone it is possible that the previous owner was running a custom ROM. In order to do this you need a modified bootloader. An incorrect bootloader/ROM match will cause constant reboot. Check your version by holding down the power and the camera key and soft resetting the phone.
See
http://forum.ppcgeeks.com/showthread.php?t=30561
http://forum.ppcgeeks.com/showthread.php?t=9093