Unless you know the seller your best bet is Swappa. You can confirm the first 3 of the serial number, which is located under the battery. The G4 is a GREAT phone. Unless it bootloops... Also, some of us are getting less battery life after MM upgrade.
I wouldn't purchase a G4 used unless you can purchase some sort of warranty on it. The majority of the boot loop problems are SN# starting between 505-508, but there's been newer ones having the issue too.v