In future you must verify that 'Find My iPhone' is switched off, that is the key to all the security. If it is on you will not be able to get around it unless (a) you have the seller demonstrate that it is off by showing you that it has been disabled or (b) have the seller go to his account on the iCloud website, then go to Find My Phone (on the site) and remove the item from his account. In your case I would guess that the first method would be preferable.
The one you have can be sold for parts value, but that's about it, sorry to bear the news.
PS: You are not the first Pawn Shop operator to post here with this problem. I don't know your business but if there is some kind of trade publication this may be a good item for it (and it is already spreading to other manufacturers devices), Galaxy G5's have a similar system, the EU will require it by law. The clock is ticking 🙂