You got ripped off, assuming that the seller has disappeared.
Activation Lock is intended as a theft deterrent, and it wouldn’t be much of a deterrent if it was trivial to bypass.
You’ll need the associated Apple ID and password either entered locally or entered remotely using iCloud.com, or you’ll need the original purchase receipt and matching ID and a discussion directly with Apple, or some legal paperwork in the event of a death in the family or such.
If this is the usual request for an Activation Lock bypass, contact the seller and have them release the lock (and remote unlock by the seller is an option) or refund the purchase, or you’re left to discuss this with the local police when that’s an option and/or to then recycle the device—this when the seller disappears, as is so often the case.
The All Too Common SAD Reality of Buying a used iPad/iPhone – Apple Community