This idea seems nice, but I'd like to expand or convince you to change this.

The reason I say this is that there are two problems I don't see being solved that could be solved in tandem with this.

1) Lack of maps

2) No need to hold additional district types (production facilities and surface research labs)

Anyway, on to my solutions, instead of just creating a market, how about making it to where corps can manufacture and/or sell weapons, dropsuits and vehicles using districts.

To expand further on this:

I believe that the manufacturing should come in two types of shipments, cartel and sanctioned gear.

cartel gear will produce more units, will sell more than the cost of producing, but will take longer to manufacture, and will have an added chance of getting "hijacked".

sanctioned gear produces less units per shipment, will sell only slightly more than the cost of producing, but doesn't take as long as cartel gear to produce, and has a very low chance of getting stolen.

How they're made:

The process starts at the surface research lab, and will end at the cargo hub.

Surface research lab role: This is where you'll produce and use blueprints of your desired weapon or vehicle. It would help if the blueprints had customizable abilities (higher damage, less recoil, etc.). The blueprints should be permanent, and should cost quite a bit if you wish to change or modify it.

Reward for capturing a hostile lab: You gain the blueprint the original owner used for your corp.

Reward for defending your own lab: This reward is dependent on if the total carnage reaches 20 mil destroyed minimum. If this happens, the defenders will receive an isk reward alongside the original equivalent to the total lost isk (divided equally amongst the defenders of course)

The production facility role: This is where I believe the lack of maps can be remedied, as I believe there should be different types of facilities for production. Four to be precise. Shipments will take, depending on type of item built, and whether it's cartel or sanctioned, anywhere from 24-48 hours to create.

Dropsuit and weapons facilities: These facilities are obviously used to produce weapons and dropsuits for the infantry players.

Turret and Vehicle facilities: These facilities are used to create vehicle hulls and turrets for the vehicle players.

Rewards for capturing a facility that is producing: If an attacker successfully captures a hostile facility with an isk lost amount equaling 20 mil or higher while it is in production, you will have two options, depending on what you have available. I will use the infantry suit as an example.

Say you capture a dropsuit shipment.

If you have a shipment of weapons either being produced or already made, you can choose to move them into the just captured facility to make a weapon and armor set with certain benefits (lowered weapon PG/CPU is a decent example). There should be additional if the weapon and armor are of similar nationality (Amarr, min, etc.)

If you have a blueprint of another dropsuit, you can cross in the blueprint with the shipment and put your buffs into the suit, to a lesser extent if they're not of the same race type.

Rewards for defending a shipment: If a defender is able to defend his shipment and destroy 20 mil worth of equipment, then he will get to use the excess salvage to put in the production, making even more of his shipment than originally before.

The isk destroyed limit should be very important, as if this not implemented, it will be abused vehemently (If I still played, I'd do it).

Next area should be the cargo hub. This is where you will hold your shipment until you either:
A) sell it on the district market mentioned.
B) put it in your own corp armory. (oh yeah, there should be a corp armory)

If you chose to create cartel gear, once it reaches the hub, it should take anywhere from 24-48 hours to sit until you can put it up on the market (sort of a let the heat die down thing). In that time, enemy forces can take this opportunity to try to raid the gear.

Rewards for capturing the hub: You'd obviously snag the shipment and keep it for yourself, but unable to sell it on the market.

Rewards for defending the hub: In the event of being able to defend your shipment, I believe getting bonus command points would be a nice little reward (if of course, the damage is 20 mil or above), aside from keeping your crap.

Well, those are my long winded thoughts on it again. Hope I helped.