We are talking about the people not going to market stalls any more after the trainer is activated. And therefor not getting their needs fullfilled.
I tried it multiple times, as soon as the trainer is activated, the people stop satisfying their needs. The market stalls are fully stocked but the people don't buy the goods. Happiness starts degrading and people leave the town. This stops as soon as the trainer is closed (Alt-Tab and X "Close trainer" . After closing the trainer the game mechanics go back to normal and the people start buying the needed goods at the market stalls.
[Edited by Lord_Asriel, 1/5/2020 3:28:51 AM]
Dunno, try it.