In followup to jmcgrogan2's post, the situation of someone receiving working equipment but then claiming it was defective by swapping in another similar but nonworking item can easily be avoided. Every sale should include a bill of sale to prove legitimate purchase. The bill of sale should include the item's serial number or other identification. If the transaction is face to face, as was the OP's, then the seller should bring a copy of the bill of sale and have the buyer sign it as a receipt to be kept by the seller. If the transaction is by mail, the bill of sale can be included in the box. In addition, the seller should take pictures of the item to show its physical condition in case of dispute about that, and one of the pictures should be of the serial number.