Hi all, I have recently started a new archipelago nautical themed campaign for my players and as many of them are more good aligned characters they have all expressed interesting in living the life of the honest merchant rather than turn to piracy for quick coin(of course we shall just see how long that lasts) so I have a few questions that I have been unable to find answers to via an internet search, the questions are as followed

1: How should I handle the pricing for mass purchases of cargo(Spices, Metal Ores, Gems, Ect) 

2: How should I adjust pricing for selling the materials at other harbor ports, as I have really only figured out persuasion checks based on player rolls and interactions with the merchants.

Really any advice that you guys have about handling large transactions of goods and running large economies, in other campaigns I have just rolled the chart in the DMG in regards to businesses run by the players. Hopefully, my question is clear but anything helps!! 

I'd make materials sell for 10% extra (depending on reputation and economy). Don't forget to charge ship fees.

I'd probably make a chart of known trading ports and relative value of good types +-10%. The last expansion of nwn2 had a trading system you might be able to find info on for inspiration.

Okay, so I'll give a couple things that come to mind as well as list a few places in the rule books to look at.
Keep in mind that in the case of finished goods like armor, weapons, tools, ale, and the like, the price is the selling price to someone. Therefore the price that the tradesmen sell it to the merchants is lower, but the price the crafter sells it to the trader is even lower.
The thing that probably keeps traders from just taking the goods from the crafter and keeping them would be the fact that traders go into debt by buying up large amounts of product, then recover from the debt and make extra money by selling the goods to the merchants.

PHB - Ch 5: Equipment - Trade Goods - Trade Goods Table
           This is a good chart for commonly traded raw materials that the players would be buying to sell at a profit.

PHB - Ch 5: Equipment - Expenses - Food Drink table
           This is a good table for some crafted items like ale that might be sold between places. Just keep in mind that items like ale are perishable and do expire. I'd search online for expiration periods of different things to see if they'd make the trip.

DMG - Ch 7: Treasure - Random Treasure - Gemstones (10 gp, 50 gp, 100 gp, 500 gp, 1000 gp, 5000 gp) and Art Objects (25 gp, 250 gp, 750 gp, 2500 gp, 7500 gp)
            This is a chart for the prices that the players could sell Gemstones and Art Objects for.

DMG - Ch 5: Adventure Environments - Unusual Environments - The Sea - Airborne and Waterborne Vehicles table
            This chart will be very useful to let you know how many crew and passengers the ship can hold. It also lets you see how fast the ship moves to judge how long they'll be at sea. And it lists how much cargo can be carried.

PHB - Ch 5: Equipment - Services - "Ship's passage - 1 sp per mile"
           Here is an average price for transporting passengers themselves.

PHB - Ch 5: Equipment - Services - "Hireling (Skilled) - 2 gp per day" "Hireling (Untrained) - 2 sp per day"
           The ship's crew will have to be paid (no one works for free.) And I expect that at least most of the crew, if not all of them, are skilled.

PHB - Ch 8: Adventuring - The Environment - Food and Water
           People need to eat. This will let you know how much cargo weight will be taken up by food and water for the players, crew, and passengers. Food = 1 lbs per day.

Google - Gallon of Water = 8.34 lbs
                I'd add a little water to make the math for all this easier and because people need some water to bath or cook with or various other things.

DMG - Ch 6: Between Adventures - Downtime Activities - Building a Stronghold - "Trading Post - 5,000 gp - 60 Days" "Guildhall, town or city - 5,000 gp - 60 Days"
            This goes over the cost and time of building structures that the players might need to make if they are running their own trading company. Also in this section would be the maintenance costs of these structures as well as the number of skilled and untrained Hirelings that each place needs. I could not find a maintenance cost for ships, but those would very realistically require maintenance too.

DMG - Ch 6: Between Adventures - Downtime Activities - Selling Magic Items
            If the players do not already have a contact for whom they are selling to, then I'd use this table to determine how long they spend trying to sell magic items AND their bulk goods to merchants and people since not everyone uses several pounds or tons of any item.

All of this will show just how complex it can be and carry the danger of going massively into debt. The players might abandon their trading lifestyle for adventuring because its simpler or because they accumulated debt while trying to live honestly. Best of luck to you trying to manage the logistics of all this stuff.

There can also be other consequences. What if pirates or bandits attacked? What if the ship took on water and damaged some of the goods that were being transported? What if the players couldn't pay their Hirelings enough and there was a mutiny? What if they became shipwrecked and lost their goods and had to adventure just to get back to civilization where they would now be in MASSIVE debt from purchasing goods and then losing them?

The things that could happen to make this become an adventure are quite high.

Bravo Golemere for your work.

GoS also has ship crew, passenger, and cargo capacity. As well as lists for random encounters, weather, and complications such as mutiny, fire, and disease and how to handle it. The book also has a system from tracking and managing the crew.

If you don't want to run an economics simulator then you could just say the PCs have the equivalent of a wealthy lifestyle without having to pay for it.

Every so often, throw in an exciting event like pirates or natural disaster, but most of the time, tell the players, "your business ticks along fine."

After all, this is Dungeons & Dragons, not Manifests & Timetables. :-) 

Thanks for all of the references! I was sure there were sections of the DMG that I was overlooking, as I have primarily been looking at the stuff in GoS since that is the most recent nautical book, but I will be sure to go back and look at all of your recommendations! 

As my campaign currently stands they are operating a single ship(themselves as captain, first mate, and so on with a few hired hands) and they feel the need to handle each transaction personally so they don't get "cheated" (of course that would "NEVER" happen, who said you can't trust a merchant right?) but as their fleet grows I did plan on transitioning over to the DMG charts for downtime/strongholds to avoid being bogged down in excel spreadsheets(in my opinion the 10th layer of hell)