Short of reaching the seller again and asking for the Ser#, are there any other suggestions for validating the device at the time of purchase?
From the link I provided earlier:
1. Turn the device on and slide to unlock.
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. After choosing a language, choosing a country, and connecting to a network, the device will begin activation. If you're asked for the previous owners Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, they can remove the device from their account by signing in to icloud.com/find. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owners account.
You will know that a device is ready for you to use when you're asked to Set up your iPhone," Set up your iPad," or Set up your iPod during the device setup process.