I definitely wouldn’t buy a “privacy hardened Pixel” from someone, it’s very sketchy. I’d only ever consider making modifications myself.
A community guide was posted on how to do it: Pixel (Tensor chip) IMEI modification guide
But note that GrapheneOS poked holes in the effectiveness of spoofing your IMEI for “improved privacy” in this Twitter thread: https://nitter.net/GrapheneOS/status/1903187370259759514#m
They supposedly also said Mudi routers are also ineffective but I don’t have a source for that which includes their reasoning.