How to recover forgotten Apple ID and password?

See this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch ->

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

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If you no longer have your iPhone or iPad

If the steps above weren't completed and you no longer have your device, use these steps:

1. Ask the new owner to erase all content and settings using the above steps.
2. If you're using iCloud and Find My [device] on the device, sign in to iCloud.com/find or the Find My app on another device, select the device, and click Erase. After the device has been erased, click Remove from Account.
3. If you can't follow either of the above steps, change your Apple ID password. This won't remove personal information that's stored on your old device, but it prevents the new owner from deleting your information from iCloud.
4. If you're switching to a non-Apple phone, deregister iMessage.
5. If you're using Apple Pay, you can remove your credit or debit cards at iCloud.com. Choose Account Settings to see which devices are using Apple Pay, then click the device. Next to Apple Pay, click Remove.
6. Remove your old device from your list of trusted devices.
7. If you purchased an AppleCare plan for your device, cancel your AppleCare plan.

I was forgotten my Apple I'd and password

It appears that your actual issues are the forgotten account information, not about selling the device

This article tells you the various ways you can try to discover your forgotten Apple ID --> If you forgot your Apple ID - Apple Support Typically an Apple ID is an email address you have used. If you are currently using a mobile device that is logged into your account, go to Settings and tap your name at the top. Look in Keychain, and in a browser's security settings if you use it to automatically sign into Apple when using a browser.

This article outlines what to do If you forgot your Apple ID password --> If you forgot your Apple ID password - Apple Support

or go directly to this link --> http://iforgot.apple.com/

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple ID password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity.

If you still need help this page provides country-specific Apple Support contact information ➞ Contact Apple for support and service - Apple Support

Try and reset your password at iforgot.apple.com. You can also enroll in account recovery there.