xeddyx

And?

By checking whether the bootloader is unlocked or not. If the bootloader is unlocked, then all bets are off, but if the bootloader is locked, you can only run the original, unmodified firmware. Any alternation to the firmware will result in the dm-verity check failing, causing the system to not boot at all. The only data which can be altered is user data, which is wiped in a factory reset. So a factory ressr definitely gets rid of it.

If you're paranoid though, you could always flash the stock firmware downloaded directly from the manufacturer's website, which will override all system partitions, so you can be absolutely sure there's no spyware - besides the spyware included by Google etc of course, or the spyware embedded into the hardware by the chip manufacturers...

Swappa is US-only, it seems that you're in the UK so forget about Swappa.

This sort of scam would only work for sellers with zero/bad reputation. Only an idiot would buy from a seller like that.

If the overall positive feedback is greater than 99% and they've sold hundreds of items then there's no problem.

I've bought dozens of phones from eBay over the last two decades, never had any issues.

Support open source projects, such as Wine, KDE, Wayland, LibreOffice etc.