Kail has stated the basics, but I shall try to give you a condensed run down, to help you grasp the majority of the concept.
Before:
There was not really much of a way to control or even tell how much stuff was getting sold on the plotgroups and where it was being sold from, and how much it was being sold for. There were the alert messages, but not really anyway to do anything with it, or change what was happening.
After:
Now we have the beginnings of a system that allows the player to take more control of plot group sales or at least to be able to more actively and easily influence it, and also know how to influence it.
They can control or influence 4 main things:
1. Number of Yields Sold e.g. Wine + Wool + Wood
2. Maximum Quantity of yields that can be sold in a turn. e.g. 10, 20 or 30 Wine.
3. Price of goods sold.
4. Which Yields they want to sell in a town.
Ways they can Influence these 4 main factors:
1. Max No. of Yields Sold is currently influenced by two things.
A. Level of Market Building (Stall, Market, Guild Hall) These each give +1 Yield Per worker. and each have an extra worker slot per level.
B. Number of workers and experts working at the Market Building. Normal citizens give +1 yield, and wily traders give +2, so a Guild Hall could sell a maximum of 6 Yields.
2. The Maximum Yield Quantity sold is increased by a number of things.
Number of Workers and Experts working the market building.
The more domestic towns connected to the plot group increases it.
If towns have (Stalls, Markets, Guilds) this increases more.
Connected Monasteries and Castles increase it by a small fixed amount (No market buildings to increase it).
Connected Foreign Cities increase it, and more so if it has a trade post.
My previously suggested quantities:
3. Price of goods is (I think) affected by 2 things right now, but more will be added (I think).
A. If a market is connected to only domestic sites, then goods are sold at half standard price.
B. If a market is connected to foreign cities, then goods are sold at the best foreign city price.
(I am not certain yet how these foreign city prices are set, I assume it uses the diplomacy code for determining goods values in trades?)
4. You can choose what yield you want a town to sell by clicking it in the yield bar, and you get the symbol appearing that shows it is being sold.
Other Misc. elements being discussed:
1. Monastery/Castle/Town Loops.
Having the three types of town connected together will introduce some kind of special 'bonus'. I am not clear on the details of what exactly this will be or how exactly it will work, but something like a price increase, or a culture or fealty boost to the selling town per sale, I think.
2. Fealty/Culture Boosts Market.
The more Fealty or Culture a City has, it will get a special boost to its trade power, maybe a percentage price increase, or quantity increase or something along those lines. So a highly developed City will be a better trade site than a tiny backwater village.
3. Buildings and Population Type/Quantity boost Sales.
We haven't really discussed this much in detail terms yet, but I think we both agree on the concept that different units and buildings will in some way affect markets. So that improving the quality of your population and buildings, will improve the income of markets.
Perhaps something like specific Units and Buildings improve the price of a specific yield. Peasants improve the rubbish goods, Artisans improve mid level goods, and Lux. Food Units like Nobles improve high end goods.
Merchant Version of Pilgrims:
I also mentioned briefly the idea of having a market building (or dock building) that would attract special 'spawned merchants or vessels' like the pilgrim, perhaps from towns with trade posts. That are attracted to these towns (like pilgrims to shrines) and when they arrive, they leave some gold and spice. (or some similar gain)
Some Randomisation in Prices/Quantities:
This was briefly mentioned by Kail, that as we develop the system, we can add a randomiser to the proceedings, so you won't always sell 10 wine for 10 gold each per turn, but there will be something like a percentage chance for the price/quantity to fluctuate in some way. This chance ca be improved or mitigated by the traders skills(future promotions system) and buildings, pop. types, etc.
I also floated this idea as a price fluctuation concept:
Trade Centre:
I also briefly floated the concept of in some way designating a trade centre in your empire, that would give trade benefits, and make it an important target in war, so if you can knock it out, you can really hurt your enemies economy and trade power.
I think that is more or less everything we discussed so far.
Kail can add anything he thinks I've missed or forgotten.
But this should give you a good 90% or more of the outcome of what we have discussed till now.