None of the carriers care if the phone is stolen, unless it's reported as stolen. They only care if it stays on their network. As a practical matter I have to work with the phone's previous owner to erase it (eg, an Apple phone that's been associated with an iCloud account, or a Samsung phone associated with a Samsung account). The carrier lock only matters after I've gone to the trouble of erasing it since I won't distribute a phone that hasn't been erased.
So far I have never handled a stolen phone as far as I know so I don't know much about that angle. What I get are peoples old iPhone 7's that have been sitting in a drawer for a few years. They are eager to donate them but have no idea how to get their personal data off them, so the work I do is sitting down with them and walking them through the process. I would say that's about 1 hour per phone including getting to and from the donor, teaching them how to reset their iCloud account password (or Samsung account, whatever) to erase the phone. When I get a carrier locked phone I sell it on eBay and buy an unlocked phone or a bunch of chargers and cables. I'm always happy when the phone is from Verizon or was bought unlocked, but AT&T is better than most.
Nice. I can imagine volunteering to do that. Or helping to wipe old hard drives or laptops. Sounds satisfying.
Do you also help teach them how to navigate the changes to each new version of iOS/Android? I would find that frustrating... so many "hidden affordances", and increasingly complexity.
