OK,thanks, your explanations with pictures clarified how it is supposed to work, but still I had the case a few times of selling less than half my existing resources of a type, and it ALWAYS switched immediately to 1/1 free lots, while saying 1/4 free lots before selecting a first sale.
I never saw the situation in your first picture, where having placed only a few items of a type the display was still at 4/4 free lots.