If the phone was sold to me and is locked what can be done

If it is a passcode lock > If you forgot your iPhone passcode - Apple Support

If it is a carrier lock > How to unlock your iPhone for use with a different carrier - Apple Support

If it is an Activation Lock only the previous owner can remove it. Send them this link > What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

You'll need to get it unlocked where it can be erased to factory settings so you can start over as if it were new. I'd see if the seller can do that for you.

Apple won't help reset it without an original proof of purchase from Apple or an authorized seller, or some other proof that it's your legal property. Other than that, you really have no options as one reason for locking is to prevent stolen property from being used. Not saying yours was necessarily stolen, but they have to operate on the premise that it might be stolen.