@AndrewK93
Ouch!!
Having to use either the QR code or the 6-character alphanumeric setup code to set up Google Nest cameras and doorbells has been a perennial challenge. I've written down the setup codes and serial numbers for all of our devices when setting them up, to avoid having to remove the camera or doorbell if a re-install is needed, but not everyone does that. You could try checking with the previous owner to see if they wrote down the setup codes or serial numbers; you need to check with them anyhow to make sure they removed the cameras from their account.
Google Nest says you can't re-install without the previous owner removing the camera from their account: "If you bought this camera new or can’t get the previous owner to remove the camera from their account, return it to the seller for a refund or exchange it for another. Let the seller know that your product is already connected to someone else’s account and it can’t be set up." (https://support.google.com/googlenest/answer/9249728?hl=en)
Google Nest does have this page, which says that if you don't have the QR code, you can select "Continue without scanning" and type in your product's serial number but I don't know if that works for cameras, and in any case you'd need to get the serial number off of the camera or from the previous owner:
https://support.google.com/googlenest/answer/9235302?hl=en#zippy=%2Chow-to-skip-qr-scanning
