Whilst I'm working on the post, I have a new Miscellaneous Ruling or two getting added to the OP. These are being added in response to the question some players have been asking about how landless souls can get extra manpower to staff armies...
Raid/Looting Clarification: It is not normally possible to loot open land or the tiny, 'invisible' villages that dot the landscape. Towns, markets, ports, temples and other significant structures may be looted or raided.
Raiding: Raiding is conducted as part of a raid by a Raider or Sea Raider unit on a significant structure, or by a normal unit on a defenseless structure. It yields 1d6 ducats, taken from the structure owner's Treasury (if any remains in the Treasury) or else from the tax provided by the structure. A structure may only be raided once per season.
Looting: Looting requires the structure to have been captured by a hostile force. Looting steals the entirety of the following year's tax income from the structure (or any remaining tax income, if it has been previously raided). If whatever structure the owner's Treasury is located in is looted (usually the capital city's fort) then thalf the Treasury will be looted as well - the remainder is assumed to be distributed elsewhere.
Looting Businesses: Businesses in a captured town or market can be looted like any structure - their income for the following year is stolen.
Dismantling Businesses: Businesses can be dismantled by enemy soldiers that capture a town, or by their owner, for half their value (i.e. the ducats invested to bring it to its present Size). Since this is highly wasteful, most owners will attempt to sell their business when they no longer require it, but raiders may be interested in the higher returns than mere looting will allow.
Private Citizen Treasuries: Citizens that own a town, market or business are assumed to have their Treasuries located within said towns/markets/businesses, or else within the capital/largest/best defended structure (capital takes precedence, best defended in second place). Other citizens are presumed to either carry their Treasuries with them or leave them in whatever town they have a house/residence in. Technically speaking, this does mean that if said town is captured or they are captured, they can be robbed of their Treasuries.
Players may opt to leave their Treasuries in other, more secure locations. This may provide a niche for characters looking to start a business based around a well-defended vault.
Capturing Slaves
Slaves can now be captured as part of a raid attempt. A raider may choose to raid slaves or ducats. Slaves are drawn from any surplus manpower in the province - if there is no manpower remaining, slaves cannot be taken. Slavers may opt to take slaves directly as manpower (pressganging them) or as commodities. On a successful raid, 1d6/2 (round up) manpower is stolen.
If they are taken directly as manpower, the province's available manpower count decreases on a success and the slaver's personal available manpower increases accordingly. If they are taken as proper slaves, each point of manpower stolen is converted into one shipment of the Slaves trade good. Unless a province deliberately operates slave pens to breed fresh slaves (i.e. it has the Slaves resource) this is the only way to obtain the Slaves trade good. It may not be obtained from normal trading at ports.
The Slaves trade good may not be converted back to manpower - breaking the spirit of the slaves also breaks their viability as a military force. A province owner may convert manpower deliberately into Slaves, but this will likely cause some uproar and potentially outright rebellion amongst the populace - provided any are left to revolt.
Edit: Adjusted the market rules and added this to the OPs. Slaves cannot be bought from any province where it is not a produced resource, or unless a slaver specifically puts them up for sale there. Thus, slaving is now a viable and potentially very profitable strategy - turning all those worthless enemy manpower points into ducats for use!