For those who sold their old iPhone and then bought a new iPhone before. I would like to confirm if the procedure I do is correct regarding files backup etc...

I would backup all my files (pics, videos) to my pc and then I sign out of iCloud and iTunes and then go to 'General > Reset > Erase All Content and Settings'. So the phone is like new, and when I COD and meet the buyer. I gave him the phone and receive money. Then I will go and buy a new iPhone and log in using my iCloud email.

Or would it be better to buy a new phone first, before selling the old one?

Thanks. I will buy new phone first before selling my old iPhone.