Astiras The problem is that "unlocked" may mean carrier-unlocked, which is not enough, and/or OEM-unlockable, which is necessary for installing GrapheneOS.
Confusingly, when buying from the Google store, "unlocked" does mean "carrier-unlocked and OEM-unlockable". I believe the same is true when buying from Best Buy.
But if a device is bought from eBay (etc.), "unlocked" may well mean only carrier-unlocked. This is particularly common for Verizon devices, which are often carrier-unlocked but never OEM-unlockable.
If it's a used device, ask for a screenshot of the developer-options screen with the "OEM unlocking" slider on the right.