Here's our trading rules:
"Trading provides a way for players to enrich themselves and to gain concrete benefit in exchange for taking an active, thoughtful, or simply money-grubbing role in the campaign.
Here's the deal: Certain commodities will be designated "trade goods".
A "unit" of Trade Goods is 100gp worth of the item in question.
Trade goods can be liquidated in the port you bought them in on a 1:1 gold piece basis. So 420 gp worth of crystals liquidated becomes 420 gp in the same port.
We could also call that 420 gold pieces worth of trade goods as "4.2 units".
If Trade Goods are brought to a foreign base, anywhere other than where you picked them up- the DM determines a haggle DC for anything you want to sell. This will be an arbitrary or random Diplomacy or Bluff DC(and usually ends up as 1d20+average party level+4). You get only ONE attempt to roll an unaided diplomacy OR bluff check. This roll represents an entire days negotiations and dealings. Sometimes the Haggle DC will just arbitrarily be set to an impossibly high number, based on the whim of the DM or 'market conditions' or for any other reason.
Success means you may sell 1d20 units at a markup of 1d100%. 1d20 units=market saturation.
Failure means that you may sell (at your discretion) up to all of your units at a markdown of 20-80% as the best possible price in the system.
Certain trading companies will provide goods on spec. Essentially you are buying goods with the promise to pay back the base cost- which can usually be paid to the office of the trade company you got the contract with anywhere in Sigil or at any of the major ports of call. This usually requires signed contracts and bonded merchant houses or consortiums.
Honoring your contracts increases your profile as a "trusted merchant"- allowing slightly increasing shipments of goods to be consigned on spec.
Not honoring your contracts lowers your profile as a trusted merchant. After a certain amount of time (say 1d3+1 months) the Trade Consortium will begin to look for, and eventually pursue ship captains to settle their contractual debts.
The ship can haul 100 Units of goods (up to 10,000 gold in trade goods) at a time. So yes, you can just travel around selling stuff. But that's when adventure happens, haha.
I don't want to do anything more complicated than this! But please feel free to make suggestions to the DM."