Here's how to remove the various types of locks...
PINs/passcodes
Typically, you can turn this off in your device's 'Settings'.
Apple: Settings > Face ID & Passcode > Turn Passcode Off.
Samsung: Settings > Lock Screen > Screen lock type. If prompted, enter your current lock screen code. Tap "None".
Account locks
iCloud
If you’re using iOS 10.3 or later, tap Settings > [your name]. Scroll down and tap Sign Out. Enter your Apple ID password and tap Turn Off.
If you’re using iOS 10.2 or earlier, tap Settings > iCloud > Sign Out. Tap Sign Out again, then tap Delete from My [device] and enter your Apple ID password. Then go to Settings > iTunes & App Store > Apple ID > Sign Out.
Samsung
From Settings, swipe to and tap Accounts and backup.
Tap Manage accounts, and then select your Samsung account.
Tap Remove account, and then tap Remove account on the pop-up.
Tap Sign out. Enter your Samsung account password, then tap OK.
From Settings, swipe to and tap Accounts and backup.
Tap Manage accounts, then select your Google account.
Tap Remove account, then tap Remove account again to confirm.
If you have Google device protection features enabled, you may be asked to verify your identity before you can fully log out of your account.
BTW – by "all locks", we don't mean your network lock
Yes, slightly confusing but we can totally accept devices locked to a mobile network.
Why is removing locks so important?
We can't rehome your device unless all your personal locks are removed. That means that we can only recycle your device and that your valuation will be severely dented.