The Marketplace is a place where Empires can buy and sell 
Strategic Resources, 
Luxury Resources, 
Stockpiles, Unit, and Heroes. The Marketplace can be accessed via the button to the right of the Diplomacy screen's button, and left of the Espionage screen's button, or by pressing F8. However, the Marketplace is locked until two technologies are researched; the Units and Heroes sections are unlocked by the Era I technology Mercenary Market, and the Strategic, Luxury, and Stockpiles sections are unlocked by the Era II technology Imperial Coinage.
Using the Marketplace[]
Resources and Stockpiles[]
After researching Imperial Coinage, it is possible to buy and sell Strategic Resources, Luxury Resources, and Stockpiles on the Marketplace. At turn 1, the Marketplace will not have much available to purchase from here, but will gradually develop a stock of most resources and stockpiles over time and from other empires selling those things to the Marketplace. Only up to 100 of a given Strategic or Luxury Resource will be available on the Marketplace at any given time, and while the same limit may exist for Stockpiles, there is very rarely more than 10 of a given type of Stockpile on the market at any given time. You may only purchase up to 10 of a given resource at a time to allow for Supply and Demand to recalculate, but there is no limit to how much you can buy on the marketplace in a turn.
To sell resources and stockpiles on the Marketplace, simply go to the "Salable Items" box in the upper left corner of the Marketplace screen. Then select the resource you wish to sell from one of the three tabs, enter the amount you wish to sell, then press "Sell". As with purchasing resources and stockpiles, you may only sell up to 10 at a given time.
TBD: Costs
Units and Heroes[]
After researching Mercenary Market, it is possible to buy and sell Units and Heroes on the Marketplace. Unlike Resources and Stockpiles that have a fairly consistent set of goods available for purchase, the Units and Heroes that can be purchased from the Marketplace are a constantly-changing roster of unique units. As such, Units and Heroes can only be bought one at a time, and once purchased, that unit/hero will be removed from the Marketplace and become unavailable for other empires to purchase.
Any units that are purchased from the Marketplace are marked as Mercenaries 
, which cannot be retrofit, even if the unit is of the same type as a unit you can normally retrofit. For example, if you have assimilated the Sisters Of Mercy minor faction and are able to produce, edit, and retrofit Justicere units, you will not be able to retrofit a purchased Justicere mercenary. However, after researching the Mercenary Corps technology, it is possible to turn armies composed entirely of Mercenaries into armies of Privateers that appear as roaming armies to other empires, and can be used to attack another empire's units in their own territory and even siege their cities without you needing to declare war with them. However, Privateers have increased upkeep, and cannot perform the Bribe, Parlay, or Search army actions.
Heroes, on the other hand, are primarily obtained by purchasing them on the Marketplace, and do not become Mercenaries upon purchase. Instead, purchased heroes are automatically sent to the Academy, where they can be inspected and assigned to a role as a Governor, General, or Spy. There are typically 10 Heroes available for purchase at any given time on the Market, with this number decreasing as they are purchased, and a new roster of 10 Heroes appearing every ~10 turns (exact # TBD). Of these heroes, 2-3 are "Exclusive" which are available to only you for a limited duration, typically 15 turns. You are the only one able to view and purchase exclusive heroes on the marketplace, and they usually have a slightly lower cost than the other heroes available.
When it comes to selling units and heroes, any units or heroes that you sell on the market will be made available for every other Empire in the game to purchase, and you will not be able to repurchase them. There are some units - such as Settlers or unique Heroes - that have the "Unsellable" Capacity, which, as the name suggests, means that you cannot sell them on the Marketplace.
Because the number of factors to take into account for a given unit - unit design, equipment, level, skills in the case of heroes - the value of a unit or hero can vary wildly from one to the next. Generally speaking, the higher the level and total 
Industry cost to produce the given unit and it's equipment, the higher the price will be. Exact equation for this TBD.
Taxes[]
While the Marketplace can be very useful as a versatile way to quickly gain some extra resources, units, or Dust, it should be noted that you cannot buy something from the Marketplace, and then sell it back at the same price. This is due to Taxes, which are a constant property of the Marketplace. Every item on the Market has a base price, and then this price is increased or decreased due to taxes depending on whether you're buying or selling, respectively. Goods purchased on the market have a 25% increase to cost from taxes, while goods sold on the market have a 60% reduction to value due to taxes.
Roving Clans[]
While all major factions are able to use the Marketplace if they have researched the appropriate technology, the Roving Clans have a much more intimate connection with the Marketplace due to a number of traits they possess.
First and foremost, the Roving Clans start with Imperial Coinage and Mercenary Market already researched, meaning they have full access to the Marketplace from the start of the game. The Roving Clans also have the ability to see all transactions that take place on the Market due to their "Insider Trading" trait, receive 8% of the Dust from every transaction made on the Market due to their "Cuts Both Ways" trait, and are able to use the "Market Ban" diplomatic action which bans an Empire from using the Marketplace at all due to their "Keys to the Market" trait, which also protects them from another Empire banning them from the Market.
The Roving Clans are also able to make much better use of Mercenaries purchased from the market for two reasons; first, they have the unique Freelance Guards technology, which replaces Mercenary Corps. The technology does the same thing - allowing the Roving Clans to turn armies of Mercenaries into armies of Privateers - but is available in Era II instead of Era IV. Additionally, the Roving Clans have the "Mercenary Comforts" trait, which grants +1 
Movement and doubles the 
Life of all Mercenary units, as well as reducing the upkeep of Privateers by 33%. These traits both serve to make the use of Privateers far more viable for the Roving Clans, which helps to balance out the fact that they are unable to declare War on empires from their "Make Trade Not War" trait.
Supply and Demand[]
While all goods on the Marketplace have a base price, this price can vary depending on how much a specific good is being bought or sold among all Empires in the game. Anytime a good is purchased, the demand for that good goes up, and so does the price; conversely, if a good is sold to the market - or it simply isn't purchased for a long time - the demand goes down, and the price goes down as well. However, this effect is not infinite, with the maximum price for any given good being three times the base price, and the minimum price being one-third.
Exactly how much each buy/sell changes supply/demand TBD.
Inflation[]
As time goes on, the base price of every item on the Marketplace increases by a small amount due to inflation. This mechanism is largely present due to the fact that, as time goes on and Empires grow, the amount of Dust they produce grows in kind. As such, increasing the cost of items on the Marketplace helps to balance things so that different goods are never too cheap.
Exact equation TBD.