I got the second hand iphone 7. It is activation lock. And i don't have apple id. Please help me in unlock the iphone.

If you are talking about "activation lock" (an anti-theft measure), 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 remove Activation Lock - Apple Support

Regards

Giulio

The seller was supposed to release it from the AppleID and ensure the carrier released from the contract.