This sounds pretty much like what we hosted two months ago.
Big market, a scheduled time and loads of people, plus attractions, entertainment and whatnot (the only reason why Call of Destiny didn't host more is that some of the big blocks are having internet issues).
So for a start, I of course agree with your views and support the idea emphatically ^_^'
Now, as it seems like we've gone through something alike maybe if I share our experience it will help in some way.
The starting time: We scheduled for a saturday at 17 GMT ourselves. Before we could have everything ready (that was like 5 minutes late or so), the place was already really crowded and the people was rushing to ask for wares. I have no doubt that starting it earlier than 19 GMT, considering it's on sunday when there are more players in-game, wouldn't mean a lack of customers.
The sales: We had three shops of our own plus the other shops/entertainment, and surely more people around than we could have expected. We had our ware lists resting at either shopkeeper's side but it became a hell to attend everybody as the question spamming was plain vile. Having lists for long inventaries is a need but I ponder if it wouldn't be a good idea to hand them to customers one by one..
The placement: Having everybody run around the whole plaza seeking for the next merchant won't amuse them and will keep people split, turning into some auction/shout spamming nonsense instead of encouraging the RP in the main tab. Somewhere more specific than "the plaza" may help gather everybody together and create a warmer ambience. Also, certain forms of entertainment may interfere with the sales and create some hassle.. you don't want that merchant's new shiny leather armor spoiled with beer ;)
Some idea to deal with the billions of Someones in such an event like this wouldn't be bad either...
Also, I would recommend posting an IC announcement
here.
Finally, is there going to be any kind of organization or is it just a
drop-your-stuff-wherever-and-sell thing?
Anyway.. I will do my best to not miss it, thumbs up! ^_^