Hi
Activation Lock prevents unauthorised use of the iPhone if it is lost or stolen.
This can be disabled only by the previous owner (who may not be the seller) either entering their Apple ID and password on the device or by them removing the iPhone from their devices list in Find My iPhone via iCloud.com.
Apple will not advise you of the previous owner's contact details, nor will Apple pass your details on to them.
If you cannot contact the seller and/or if you believe that the iPhone may be lost or stolen, then I suggest that you hand it in to the police.
More information and instructions:
We have less of an idea than you do.
Figure out who took your money, and contact that person/organization.
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