tvamerica Then how can I factory reset the phone, so I could 100% sure that is fully clean?
A factory reset might not be enough, you'd also want to verify the OS hasn't been modified. You can restart the phone and while it's starting up, make sure there's no notice saying the phone is running an alternate OS. If you don't see that, I think a factory reset is sufficient.
tvamerica Is it okay to buy a used Google Pixel from the ad site (something like eBay or Craigslist)?
Make sure you confirm with the seller that the bootloader can be unlocked. Sometimes locked phones make their way into other countries.
If the seller doesn't know if the phone's bootloader can be unlocked, you can ask them to check if enabling OEM unlocking works from the phone's settings following the Enabling OEM Unlocking instructions on the website.
tvamerica is it okay to store this information on a used phone, before I transfer it to a new one?
Should be fine as long as you confirm the phone is running the unmodified Stock OS (as I said earlier) or if you flash GrapheneOS, disable OEM unlocking, then verify the installation.
tvamerica Also, because I'm not going to use it much, is it okay to buy an older model? I just need it to function for a year, and that's all.
The oldest recommended devices are the Pixel 6 and Pixel 6 Pro. The 5a is still supported, but won't be supported by Google for much longer.