Let's not bring consumer demand, supply, and all that into a discussion about the fort's internal stuff. For certain things, namely necessities, it's one thing, but for stuff like ores and whatnot, that's another.
That's a whole horrific can of worms we don't need reopened, and far more complicated than what would be fun to work with. Something like that is all well and good for a grand scale situation, but not the relatively small scale of Fort mode and a possible extension for the immediate area around it once you have a landholder. Our focus is simply too small for stuff of that nature to matter for us (or, in my opinion, not really a fun inclusion, just needless complication to intra-fortress activity.)
If Shop A has Item B and Dwarf C has money, he'll buy the item and get a happy thought/a stress reduction. If the item happens to be made from something that Dwarf C likes, even better modifier because of material preference. If the item is a necessity (shirt for example,) they'll get a happy boost because they won't have to buy it later/depend on government stock.
Dwarves with enough free time may opt to buy a shop if there are any available, and buy it from the fortress (basically buying it from the Mayor/Noble,) and buy a random assortment of goods based on the preferences of dwarves they know + thier own and material preference. Can limit what a given shop can sell by using the Manager to set it: Clothing Store (only sells clothes,) General Store (sells whatever,) Craft Items Store (sells crafting items such as toys, instruments and amulets.)
During break times or sufficient idle time, dwarves will go to the shops and buy items from the shop owners. Likewise, they may use public workshops to make items based on preference (after buying the materials from the Mayor/Noble,) to sell for a small profit later (be it to a fellow citizen, a visitor in the tavern, or a caravan,) or even keep for themselves. Might necessitate a "Private Stocks" screen possibly tabbed to from the "Fortress Stocks," to keep track of any privately owned weapons and armor in addition to other 'private' goods.
Things like weapons, ammunition, and certain tools such as jugs would be barred from sale to private shops, because those things are "vital" fortress items, restricting private weapons and armor to caravan and between-dorfs purchases or making the gear themselves.
I can't really say anything on the matter of food other than prepared meal prices desperately need to be changed. All for "raw" foods like meat and basic crops like plump helmets or squash being either free or dirt cheap so the population doesn't become a bunch of emaciated zombies.