Also, rather than just making the buyer refuse to buy if he has STOCK > 10 or whatever, how about if he remembers how much he bought and sold of each thing in the prior period? Let's say every week.
Frontier example.
A shopkeeper needs to sell pelts to a traveling trader. The shopkeeper is the middle-man. The trader is the conveyer back to the civilized country, which is outside the scope of the frontier. The player fits into the position of trapper.
So it looks like this: Trapper -> Shop -> Trader -> Homeland.
The homeland has a demand, let's say for 1000 pelts per week. It resets every week, so if you send nothing this week then the homeland won't want 2000 next week ... still just 1000 in any week. That means the trader knows he will be able to sell as many as he can get, up to 1000. He will have no reason not to communicate that to the shopkeeper. The shopkeeper knows he will be able to sell up to 1000 pelts per week to the trader.
Let's say the shop sold 500 pelts last week. The shopkeeper will think, "1000 is the limit to my inventory, otherwise pelts are sitting around in my shop unsold until the next week - and what if I don't sell all of those either? Best to stick with a max of 1000 pelts at any time". But he also thinks, "I successfully sold 500 pelts last week, so I can have at least 500 in my inventory." He will want to buy between 500 and 1000, because he wants to maximize profit, so he wants to sell as much as he can manage.
If next week he sells 600 pelts but retains 100 in inventory (because the trader bought only the 600), that's inactive capital. Now next week he's concerned - will he only sell 600 pelts again? If so, he wants to get 600 to sell, but any excess are a risk to him, so he should pay less. 600 is now his "soft cap" because he retained unsold pelts, and 1000 is his "hard cap" because he knows he can never sell more than that to the trader.
But of course if you offer him pelts at $1 each instead of $10 each, he will probably buy all the pelts you can sell him because he will eventually get to sell them onward. At what point will he be unwilling to continue buying extremely cheap pelts? Probably when he runs out of funds to buy pelts and must wait for the trader to buy some.
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So each week your player, the trapper, comes to the shop to sell pelts. The shop will have a current inventory, let us say 100 pelts. It has a "soft cap" of 600 pelts, which means you can sell him enough to raise his inventory to 600 at the full $10 price. After that he expects a discount, perhaps down to $4, up to his "hard cap" of 1000 pelts. After that he expects a harsh discount, only $1 per pelt.
At some point the trapper will be unable to sell all his pelts. He will probably turn his attention to other pursuits - perhaps charcoal-burning, or smoking meat - because he knows producing pelts in excess of the shop's "soft cap" is a less valuable use of his time.
The trader who buys from the shop, of course, will buy as much as he can carry - he wants to maximize profit by carrying as much as possible - and travel to the homeland to sell. He will use capital to do this, in the form of donkeys, a cart, a rifle to protect himself, perhaps guards, and the food etc. needed to support himself while traveling. He arrives and gets his profit from the sale, and returns through the wilderness to the frontier to purchase more pelts. As he becomes more prosperous, he is able to buy and sell more pelts at once, making the shop's "soft cap" increase, to as much as 1000/wk.
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The trader may lose his shipment. This puts the trader at a point where he may be unable to purchase another shipment of pelts from the shop. He may go to the shop and ask for pelts at a lower price, because he can't afford to buy enough. The shop might agree, or might offer to lend to the trader, which is a separate issue. Or the homeland may offer the trader less per pelt, which means the trader will want to pay less per pelt. Either way, if the shop receives less per pelt, it must offer less per pelt to the trapper. This probably affects the base price of $10, which in turn reduces the "soft cap" price from $4 and the "hard cap" price from $1.
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Now what if we have multiple people in each category? All the traders are vying to fulfill the homeland's 1000/wk demand. All the shops are trying to sell pelts to these traders. All the trappers are trying to fulfill the shops' demands up to their "soft caps". And everyone needs to do it just before the other guy gets there.
That means trappers are in competition with each other, just as shopkeepers are, and traders are. Although, instinctively it feels like shopkeepers probably have less pressure to buy from trappers, and more pressure to sell to traders. That means the traders have relatively more power in this scenario and trappers far less power; also traders have the most to lose (their entire livelihood - their method of making money by using money) while the shops regulate their own risk by restricting pelt-buying, and trappers who cannot sell lose only their time spent gathering pelts. Besides that, of course, if everything doesn't work out perfectly and trade cannot continue, everyone will starve to death.
Because traders tend to boom and bust, and only traders and shopkeepers make significant money in this enterprise - and especially shops because they can perform work far in excess of time constraints, only restricted by money restraints - shops should gather the most money under them as a class. This means if a trapper or trader needs financial help, a shop can lend to him to continue the economy. This continuation benefits everyone, but the loan also nets the shop interest, propelling shops as a class farther ahead of the others economically.
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This is all coming out of my butt here, so if anyone wants to chime in on how I'm clueless, please do!