Caseback on a Frog is a must! First to admire the back itself and second to check the serial and work out production date. Yes, specs copied from a Casio site are not always necessary - the bare statistics maybe but so many copy Casio's blurb as well.
What does constitute a good ad? To me clear, sharp pictures, description of any cosmetic fault, date showing on the watch fairly close to advert date, an invitation to ask questions and sensible postage.
Warning signs for me are: poor quality pictures especially of models prone to being faked, stock pictures instead of the particular watch for sale, poor description of the watch, cut and paste of the watch specs (as rutteger mentions above) without further description and excessive post and packing charges.
Anybody else got any pet hates on eBay listing?