1. Small box, watch inside securely with bubble wrap or other packing material. Tape seams.
2. Place small box in outer box, preferably at 90 degrees so the major seams don't align. Tape seams.
3. Wrap outer box in brown paper, to mask where box seams are. Tape seams and all edges to reinforce.
4. Photograph the whole package and send to buyer. They can then easily check for tampering.
5. If the buyer has the photos at receipt, they can check the box whilst the delivery person is present before accepting it, flagging up any problems immediately.

Might seem like overkill, most go through easily, but it's a worthwhile form of (figurative) insurance.