No one can help you with this, other than the person holding the Apple ID. They’ll either need to recover access to that Apple ID, or provide original purchase receipts for Apple ro unlock it. Otherwise, it’s fodder for recycling. And given it’s an iPhone 5s, it’s already a candidate for recycling, as it’ll never get past iOS 12.
Activation Lock is intended as a theft-deterrent mechanism. A theft deterrent that is easy to bypass would not be much of a deterrent.
To clear an Activation Lock, you will need the password of the associated Apple ID, or the original purchase receipt and a discussion with Apple Support, or similar proof of ownership.
If the seller is still available, contact the seller for remediation or for refund. A remote unlock using iCloud is available to the person with the Apple ID holding the Activation Lock, if that person is still available and is inclined to unlock this device. And remembers the Apple ID and password.
Again, only the person with the credentials to the Apple ID holding the lock, or Apple, can clear this lock. For Apple to clear the lock, you will have to provide a receipt with proof of original purchase and with matching serial number or IMEI, legal paperwork from an inheritance, or other similar proof of ownership materials.
For recovering access to the Apple ID:
If you forgot your Apple ID password - Apple Support
If you forgot your Apple ID - Apple Support
Apple’s I Forgot Service
How to use account recovery when you can’t reset your Apple ID password - Apple Support
Contact Apple for help with Apple ID account security - Apple Support
Purchasing devices with Activation Lock is unfortunately far too common. These are scams, and other fraudulent sales. These locked devices will not be useful to you, and are probably best recycled. See the following link for more info:
The All Too Common SAD Reality of Buying a Used iPad/iPhone