no original sale receipt

The original owner would need to follow the steps to transfer the AppleCare to your name. I’ve done it twice for iPhones I have sold when upgrading. https://support.apple.com/en-us/HT202712

Are you saying the used phone you purchased has AppleCare+ or the standard AppleCare one year warranty? In order to be able to buy AppleCare+, it would have to have been purchased within 60 days from the date of original purchase of the phone.