Yes, but that is incredibly risky and I would argue a worse proposition for one's personal data than buying an Android device or iPhone direct from manufacturer. You don't know if any of the underlying system software components are compromised from a reseller. If you bought a used Pixel, reformatted the storage, and installed GrapheneOS fresh, you can be confident that the OS is untampered.

I would go as far as to say buying a preinstalled device would never be a good idea for any individual looking to use GrapheneOS. The tutorial for the installation is confined to a single webpage. People should learn to read the instructions and use the validation tools on GrapheneOS's site.