Two brief notes
1 Though the reading for 'esun' is, inter alia, 'shop', the underlying notion seems to be 'trading' not 'shopping'. So, think first of all of a bazaar, not a mall, and then see how much of that carries over into the world in which you live (and how little of it is in stock trading, for example).
2. 'tenpo esun' doesn't have to be seven days, but let us take it as that conventionally. How do we know when seven days are up if we can't count above two? Well, for one thing, God hung the moon just for that purpose (it says in holy books of the west and middle east): the visible halfway point between full and new and again between new and full comes to average out at about seven days -- and if the market is earlier or later by a day, what the heck. Folks in Lao-lao- land notice stuff like that (and can actually see the moon and some stars).