What it would be
Three documents that a shop already needs and mostly improvises.
A letterhead, for the correspondence that still happens on paper: a condition report, a valuation, a letter that accompanies a service.
A packing slip, which is the one that matters. A watch travelling between two businesses arrives in a box that somebody opens, and a slip that states plainly what was sent and what condition it left in is the difference between a disagreement and a conversation.
A note for the box. Short, unbranded on the front, the sort of thing a buyer keeps in the drawer with the papers.
What has to be settled first
Whether the documents carry the Worldwatch mark at all. A dealer's letterhead is their own, and a supplier's logo on it reads as a franchise rather than a membership. The likely answer is a small verified line at the foot and nothing else, but that is a decision someone has to make rather than a default to fall into.
The kit overview lists the rest of what is proposed.