Send unneeded family members to Trade Missions (under 3k gold per one) until it tells you there are no more ports to trade with. This unlocks multiple markets in the Port building.

Dedicate a character to trading - maximize her Bonus to Bargaining.

• Pregnant woman (gets a huge +10 to bargaining for some reason)
  
• Spend skill points for Charisma
  
• Train charisma (gives +1 temporarily, does not stack with itself)
  
• Free citizen clothes (or the slightly weaker Stately garb for raw charisma or slightly stronger but costly Gorgeous clothes)
  
• Money bags / Slide rules / Lucky stones - anything that gives +bargaining
  
• Social influence upgrades (if you have the titles and influence to spend)
  
• Merchants guild (might not be worth the requirements to join and stay in)

Always buy and sell with this character. You will see that they get offered better prices even before buying/selling the items. But only directly from/to their inventory, not e.g. henchman's.

Example:


Wait for a Quest called Long-distance trade opportunity. It will specify a port that either sells goods or buys them for a good price. Send your trader there.

With your trader at port and one port having the quest event "long-distance trading" and at least some capital:

1. Optionally set time to slow motion - no need for time to even flow
   
2. Switch to products of the quest category (what is currently cheaper or costlier e.g. Trinkets or Luxury items)
   
3. Switch between markets in the port and find items with good buy price in one and good sell price in another market
   
4. Buy for cheap in the cheaper market and immediately sell to the other that buys for a better price
   
5. If you sell everything quickly just wait and stocks will refresh partially (not in slo-mo, d'oh)

Prioritize cheaper items so you can buy enough of them when starting. Then go by the price difference to not damage the price of costly items by trading in small quantities. Should not be a problem in slo-mo tho.

Alternatively you can sell to a Market if it offers a good price. I sold 150 Lucky stones this way, which later made them extremely cheap from that market, so I bought them and sold elsewhere again :D

And with good enough Bonus to bargaining, you can even buy/sell for the same price and still make a profit.

This is how the result should look like: