What should I know before buying a used phone?

Episode 1557 (14:23)

Mark from Tampa, FL
Smartphone and laptop

Mark bought a phone on eBay and wants to know if it will work with his carrier. Leo says that it may, but it depends on the radio frequencies it supports and whether or not the phone has been locked to the carrier or not. You can get the carrier unlock it, and they should do it as long as it is in good standing. He'll also need to make sure the phone isn't carrier locked.