Here are photos from the Piazza San Marco (including the Christmas tree) -- you can see how deserted it is -- this was around 5:30pm -- as well as the interior of San Marco. As mentioned before, despite guards being throughout, no one was enforcing the no photography policy. I am beginning to think they do so usually because of the huge numbers of visitors. But as you can see, that was not a problem.
I never saw it like this before because there were always hordes of tourists. There were less than 5 people in any section of the basilica at any given time I was there -- something truly remarkable! I think I may confine most future trips to December and January, where I've enjoyed great prices for hotels and now more of the real Venice.
You're always taking a chance with weather in Venice, but then perhaps that's part of the fun! I just checked the meteo on Il Gazettino and the weather seems to be horrible now -- yet I had spectacular weather. A roll of the dice, though I wouldn't (and couldn't) go in November.
I haven't noticed it as much in Italy (since you have fewer traditional department stores or ones like them, but in France the big sales (soldes) are in January and August. Murano tends to be less susceptible, and I haven't figured out their patterns for sales. I think it may be more based on what designs/creations have been selling and what have not.