The buyer is arriving tomorrow and the photos have finished copying. There is one more job: make sure the old phone is no longer the only way into your accounts.
Transfer, then check
On your replacement device, open a few important files and sign in to the accounts you use regularly. Check your password manager, authenticator and recovery options separately. Having the application icon on the new phone does not prove that its contents or credentials transferred.
If you use a mobile plan or eSIM, follow the provider’s transfer instructions. Keep the old device until the checks that require it are complete.
Use the right handover procedure
For an iPhone, Apple provides a sequence covering backup, transfer, account steps and erasing the device. Follow the current instructions for your model. Other phones need their own manufacturer’s procedure; button names and account protections differ.
Do not improvise by manually deleting photos from a synchronised library while still signed in. You may be changing the shared library rather than merely clearing this phone.
Inspect the result
After the erase procedure, check that the phone is ready for initial setup rather than opening your home screen. Resolve any remaining owner-account requirement before handover, using the official process. Remove your physical SIM if applicable. Keep your backup and transfer notes until you have used the replacement device long enough to notice anything missing.