Just coming to comment that I have also done several things to make sure my game and saves are clean before testing the Street Gallery and Come and Get It Street Store items. Repaired the game, reset the Sims 4 folder, except I did not add back any of my previous folders, I started the game as if it were as new as the day I bought it (which also means there's no CC). Tested both tables in a new game, and did not sell one single item. The sims are clearly trying to shop, but then it seems (to my experienced game play eye) that they are prevented from making a purchase, so they proceed to idle on the lot. After idling for a short time the sims will come back to the table and shop again, but the same thing happens again when they are prevented from making a purchase. It's a continuing cycle until the sale ends.
In my amateur speculation and previous experience with these tables, the game will send 1-3 sims to the lot and run a shop/purchase cycle, and then sends all those sims away, then the next batch of 1-3 sims will arrive, and another shop/purchase cycle runs. This continues steadily until the end of the sale - unless the cycle doesn't complete, in which case the group of sims holding up the show stay on the lot until the sale ends. Unfortunately when this happens, no new sims arrive and you end up not selling much. (this happens in about 1 out of 3 sales, so it could be one of those intended random chances of it being a good sale, vs. it being a dud, I don't know). With this bug however, since the shop/purchase cycle doesn't complete, the first group of sims to arrive never leave until the sale is over, so no new sims arrive, and no sales are made. At least, that's how they behaved in my game.
I hope this information is useful for someone to pinpoint what needs fixing, it truly does seem like the sims are shopping, but just can't complete the purchase. I blame the tweaking done with Retail shopping, it seems like every time the retail engine gets tweaked, it breaks the yard sale engine, and vice versa. Can't the two just get along? :D