Shipping: Like most internet shopping, if you need to return something it is usually on the returner, not the seller. Shipping is one of those "cost of doing business" kind of things for both parties if the situation arises. IMHO shipping costs is a "shared" responsibility.
I have always viewed purchasing reps a lot like gambling. You win some, you loose some (in terms of expectations of the product).
I think Panerai153 had some great ideas. Photo/Video the whole process going into the package. It is a two-way street. If I sold something, and the buyer opened it to regulate or something and screws it up, it is on them. Maybe as sellers we need little "void if removed" stickers on the caseback. That way the buyer can wear it for a week, and once he removes that sticker there is no recourse against the seller.
I have had one DOA arrival, most likely damaged in shipping. The seller refunded me. In mutual respect, you could follow the timestamps of the tracking, and notifications (this happened to be UPS). The watch was in my possession for less than 10 minutes. (Delivered at 3:00, new shipping label for its return processed at 3:10). I ate the shipping costs.
Back to the COSC thing. IMHO, if the watch is within ONE minute per day (5 minutes by the end of a work week) that is OKAY and probably doesn't require a disclaimer. If the watch is running worse than that, a nice "heads up" in the sales thread would be nice. i.e. watch is running 90 seconds fast per day.