Thought y’all might be interested in a House Rule I’ve instituted. Every seventhday is a Market Day, during which time the market class remains the same but all goods and services available are increased by 50% (this includes the percentage chance of finding certain goods). Therefore, if there are 10 items available normally at a certain price point, there would be 15 available on Market Days. Likewise, if there is 50% chance of a certain item being available, there would be a 75% chance of finding/selling it.
Every fourth Market Day sees the market class increase by one (1) and is a major feast day. For instance, every fourth seventhday sees a Class VI market increase to a Class V market, as caravans arrive, folk stream in from the countryside, etc.