This is called iCloud locked or activation locked. You bought an iCloud locked iPhone. No one here will help you try to circumvent that.
You need to contact the seller and get them to remove the device from their iCloud account. If they won't do it, then you need to pursue a refund.
Next time you buy a used iPhone, check its activation lock status here:
https://www.icloud.com/activationlock/
Also, read this:
http://support.apple.com/en-us/HT201365
and this:
http://support.apple.com/en-us/HT201441
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In all fairness some of these people likely are naive buyers, not thieves. A better response is the one I gave which will educate them for future purchases.
People on this site may be very educated about these things, but the average person is not and they can easily be duped into buying a phone not knowing this could happen to them. Lots of honest people get taken advantage of by thieves and scammers. If a naive buyer doesn't understand why the phone won't activate, I can see someone legitimately asking for advice on what to do. What they should do is get a refund if possible and if not, chalk it up to an expensive life lesson and learn how to prevent if from happening to them again in the future.