As mentioned before, sales are from the previous month.
It is possible to sell snow shovels in spring (month 4) and thus the sales will be recorded in the summer month (month 5).
It is possible to sell up to the maximum amount of a product any month where the demand is not zero.
From you description of selling snow shovels, it seems as if you can sell 40 units maximum. The demand in the month and your price determine how many snow shovels you can sell. If you product is competitively priced, you can sell 40 units if there is some demand. If it is not competitively priced, you may have problems selling 40 units even in winter.