I forgot the ID after buying an iPhone 7

See if you can recover the ID/password.

Apple ID - How to find your Apple         An iCloud e-mail address is an Apple ID.

Apple ID - Find

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage

Apple ID – Forgot Password

 if you are the original owner, take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store.

Genius Bar Reservation US

or contact Apple ID Support. They will need to see the original purchase receipt/paperwork also.

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat         Select your device, Battery, Power and Charging, then Power adapter not working to get a chat option. 

Apple Support using Twitter

Or fill out and submit this form.

You are welcome. Did you try all the email addresses you have/had before?

You are welcome.