Thanks for a very good post Patrick! Along with many decent people that populate the Internet (and the real world), unfortunately the small percentage of those who are either incompetent or downright criminal is always, potentially, around the corner.
I'm sure this specific C24 ad was made out of (an incredible amount of) incompetence rather than bad faith. But still, since the end result is very similar for a buyer, even one of the mistakes made in that ad is entirely unacceptable on a reputable platform.
I've been a religious follower of rule #3 so far, namely I always bought new from ADs, which is obviously the safest way to go. I don't rule out buying second hand, in the (unlikely, for the watches I look for) event it will be the best way to arrive at a certain piece. In that case, I would definitely agree with your approach, and with many of the suggestions you listed. Buy from a friend, or "buy the seller" is always another good one to offer.
The only thing I'll never resort to is hiring experts in order to buy a watch...for me, this would totally spoil the joy of what I view as a fun hobby rather than a dangerous business. When the stress or risk of a purchase surpasses the fun, my best suggestion for anyone is always to pass on that watch altogether! ;-)
Advice #10 is great and, in fact, is what I tell student at the beginning of *every* course I teach: Never trust what I say or write on the board, and if I don't give you evidence for my claims, then don't believe me!
Great they teach this in MBAs as well, as that is a point I make in every single math class -- from 1st year undergrad all the way to PhD topics courses!
Frankly, if everyone did the same (obviously, to the extent this is feasible in a given discipline), I think academia as a whole will be a tad more rigorous and believable...not to mention, as a consequence, that our students who grow to become watch enthusiasts will fall for fewer scams during their collecting careers!