Buying used Pixel phones is an option if you can make sure that:
- the phone is coming from a somewhat trusted source and does not have major physical damage
- the phone is not registered as stolen or otherwise has its IMEI blocked
- the phone is "unlocked" in the sense that it will accept all SIM cards
- the phone is not carrier branded, i.e. it is not a Verizon variant where the bootloader cannot be unlocked (this is much less of an issue in the EU)
- the phone is actually cheaper than a new phone (check for bargains)
So in your case, you should make sure that the phone is not only "unlocked" (referring to it accepting all SIM cards), but also not carrier branded, as the latter will lead to problems with the installation of GrapheneOS.