I was sold a locked iPhone

If you are talking about "activation lock" (an anti-theft measure), AND if you cannot remember or don't know the old Apple ID & Password (even using these instructions:--> If you forgot your Apple ID email address or phone number - Apple Support or these:--> If you forgot your Apple ID password - Apple Support ) neither we, just users like you, nor Apple can help you, sorry, unless you are the original owner and you still have the proof of purchase (receipt/invoice reporting your device’s IMEI or Serial Number). Should be this case, start your request using this link: https://al-support.apple.com/#/additional-support

Shouldn’t be this case, activation lock can be removed only by the previous owner, with the credentials that were used to activate that iPhone/iPad. Without those credentials, that device is useless to you.

You can also take a look at this support article:--> How to removeActivation Lock- Apple Support

Regards

Giulio

Return the phone for a refund. Other than a paperweight, it is useless unless you can somehow find the original owner and convince them to release the lock on the phone.

Apple will not help you with this.

Talk to whoever you purchased the phone from. Apple can’t help you.