Commodity shops are special shops, run by and located in the places that produce raw materials (commodities) such as iron, silver, steel, leather, coal, cloth, and so on. These village shops each produce X quantity of commodity every X period of time, and it is generally different for each village and each commodity. No village produces all of the commodities, and you'll find that prices will vary from village to village according to the available supply: As the number of commodities in stock dwindle, prices will rise. Conversely, if there is a large surplus, the price will likely be lowered. This creates the possibility of stocking up when prices are low, and then selling them via your own shop when prices have gone up.

Note that certain commodity quests can be performed to help a village produce more of a commodity (even if it normally doesn't produce that commodity). For instance, bringing live chickens to the chicken coop in Stewartsville will cause Stewartsville to produce more egg commodities. If you kill the chicken and sell it to the butcher, on the other hand, the village would produce more poultry commodities.

See also: HELP SHOPS