This is the least interesting article on the site and the one most worth reading, because getting it wrong is the single most common reason a phone sale stalls.
The order matters. Back up first, sign out second, erase third. Doing them in the wrong order either loses your data or leaves the phone locked.
Step 1: back up, and check the backup exists
Settings, your name at the top, iCloud, iCloud Backup, then Back Up Now. Wait for it to finish and check the date it reports afterwards. A backup that says it last ran in March is not a backup of your phone today.
If you are short on iCloud storage, plug the phone into a Mac or PC and back it up there instead. Apple gives you a free temporary iCloud allowance when you are transferring to a new device, which is worth using if you are upgrading rather than just selling.
Do not skip this because you think everything is in the cloud already. Photos usually are. Messages, app data, WhatsApp history and Health data frequently are not.
Step 2: transfer anything tied to the hardware
A few things live on the phone rather than in your account, and they are the ones people forget until afterwards.
- Unpair your Apple Watch from the Watch app, which backs it up at the same time
- Move or delete any eSIM, under Settings, Cellular
- Transfer two-factor authenticator app codes to your new phone first
- Remove cards from Wallet if you are not erasing immediately
- Sign out of WhatsApp and transfer the chat backup
Step 3: sign out and erase, in one step
Go to Settings, General, Transfer or Reset iPhone, then Erase All Content and Settings. On any recent iOS this signs you out of your Apple Account and removes the Activation Lock as part of the process, which is why it is the recommended route rather than signing out separately first.
The phone will ask for your Apple Account password. That is the Activation Lock being released, and it is the whole point of the exercise. If you cannot supply it, stop here and recover the account first, because the phone cannot be sold until you can.
When it finishes you should be looking at the Hello screen. That is what a phone ready to sell looks like.
What if the screen is broken?
You can do all of this remotely. Sign in at iCloud.com on any computer, open Find Devices, select the phone, and choose Erase This Device. Once it reports as erased, choose Remove From Account.
That releases the Activation Lock without touching the phone. It takes about two minutes and it is the difference between a broken phone that is worth money and a broken phone that is worth nothing.
Step 4: pack it sensibly
Wrap the phone in something soft, put it in a box rather than a padded envelope, and fill the space so it cannot move. A phone that rattles around inside a box for two days arrives in worse condition than it left.
Include the charging cable and the original box if you still have them, because both add to the offer. Take a photo of the phone before you seal the box. Nobody has ever regretted having that photo.
The short version
Back up, check the backup, unpair anything attached to it, then erase from Settings so the Activation Lock releases. If the screen is dead, do it from iCloud.com instead. The whole thing takes about ten minutes and skipping it is what turns a three-day sale into a three-week one.