A few observations:
Total firewood capacity in the marketplace is one firewood per burgage plot. That means if you have 20 burgage plots, you will only ever see 20 firewood in your marketplace, max. That's why you may be seeing empty stalls.
Total clothing capacity in the marketplace is one type of clothing per burgage plot. That means if you have 10 burgage plots and 50 leather in your storage, you'll only ever see 10 leather max in the marketplace. This is where variety comes in handy here. If you also have 50 linen in storage, you will then have 10 leather and 10 linen in your marketplace, effectively doubling your ability to keep your villagers' houses needs fulfilled.
Total food capacity in the marketplace is one type of food per burgage plot. If you have 5 burgage plots, your marketplace will only every have 5 max of each food type in stock. Given the priority of food eaten, if you always have meat, berries, bread, and eggs in stock, your villagers will never eat veggies, apples, or honey.