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's 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 demand: If a particular
commodity in a particular shop is sold out, the price will likely get raised
when it is restocked. Similarly, if there is a large surplus, the price will
likely get 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