I recommend before even connecting a used iPod to go to 'Settings', 'General', 'Reset', then 'Erase all contents and settings'.
I once bought a used iPod Touch and did a full iTunes restore. Everything on it erased. I then downloaded Pandora, to my surprise, when I opened Pandora, the other persons account was there. It didn't even ask for a password. All their stations were there and when I checked the Pandora settings, it had that persons email address and it was the same person that sold me the iPod! Weird!