Security when buying a second hand PinePhone

[tophneal]

it's likely not something you'll really need to worry about (that's a whole lot of hassle to muck around in the modem firmware just to get rid of the phone,) but if you feel inclined, you can build and install firmware yourself: https://github.com/Biktorgj/pinephone_modem_sdk
links and instructions to flash to stock can be found here: https://github.com/Biktorgj/pinephone_mo...ECOVERY.md and there is documentation for interacting with the modem firmware in the wiki.

i don't believe there's any other firmware that's OS-independent you really need to worry about. A fresh OS install should settle any fears, and void any need to wipe the internal storage, as writing the OS img will wipe out any existing data.