Normal commerce as we understand it - where things have set prices - is the exception through history. So buying and selling stuff is often a feat of relatively random bartering. Happily, we have a lot of dice to simulate that with.
Shopping
When you are wondering through time living out of a single bag (if even that) shopping might easily become a crucial bit of survival. Shopping with the aid of some sort of organized collection of data (like, say, a cataloger) is an IQ task. Shopping in the crowded bazaar is a PE task.
Shopping takes place in 1 hour turns.
This can all be skipped with a Fate Point.
The initial difficulty in finding an item for sale is determined by the relative scarcity of the item.
A secondary consideration is the cost of the item. Since money (even the old gold coin) is a floating concept, we go by median prevailing wage for the locals.
The default location is a village up to a large town.
Also:
Selling Stuff
Finding a buyer is every bit as difficult as finding a seller, and the factors apply - mostly.
Advertising (it could happen) could subtract 1-4 DD.
Otherwise, The Difficulty is the same as Shopping.
Haggling
Once buyer and seller have found each other, there is usually a negotiation. (The modern American practice of fixed pricing is a historical anomaly).
The deal is trading one thing for another, or more likely one group of things for another group of things. How that works out depends on too many factors to lay out here. The XP will have to work out some sort of base price.
If no other means can be worked out, try this: the difficulty from shopping for the good is the number of common items (that the seller would plausible want in trade) it takes to trade for it. So, a longbow, which would be VH by default would cost, at a minimum 4d6 arrows.
Haggling is a contested action, pitting each party's Core+CH plus any relevant Focus or skill..
Here are some modifiers:
The party with the highest total roll can subtract the remainder from the final price.
EXAMPLE: Brass, in Luhuang China circa 825 AD, wants some extra bow strings for her bow. This is VH by default, but +1DD because it is more than an hour's wage, but -1DD because Luhaung is a city at this point in history, but is +3DD because Tang dynasty China is TL-6. So that's a total of 7DD [rolls 18]. Brass only rolls 5 dice for this, but the XP gives her 6 dice because she's an expert archer and knows what to look for. She rolls 22 and finds some [The XP rolls a dice and says the bowyer has up to six.]. Brass is offering a silver coin (a day's wage) in exchange for the six of them. They haggle.
(The merchant's shop bonus is cancelled out by Brass's cash bonus, so they don't roll either dice.)
Merchant: 3d+1[CH]+2[Focus] for 6 dice [26]
Brass: 4d+[-1CH]= 3Dice +4(the remainder from shopping) for [13+4=17]
The XP rules that a silver coin is normally worth 10 bow strings, but with a difference of -9, she only gets the one.