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I'd prefer to leave this until v4 becomes live, but ATM (other than the standard obvious buy low, sell high - & get your trades in first), smalltalking multiple NPCs to gain stackable discounts can work effectively. if a little tedious..
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As above, but there may be a few traders on the station that have different prices for their wares; worth checking if you want to buy a bunch of green marines or sell off your illegal wares from a distance! There is no transport fee for /selling/ but there is one for /buying/ - see below
Black market trader is traders are not available to view in menu the general Menu <Enter-5>, but still can be found in the Dock NPC list that opens in details screen and can be contacted remotely. You can (if selected) when you open the Details screen; therefore you can sell illegal wares while out of reach of police ships, remotely.
All trader prices and inventories are randomizedrandomised, so if you see 2 traders of the same type, they may have different prices. Sometimes you can buy from one and sell to another for profit without leaving the platform or station.
Each Every remote trade adds a 2500 Cr transport fee. <Check whether this is per item type or per total transaction. Does it apply in the same way to remote capship crew hire? - Snafu>
High reputation with the trader's Faction will give a discount on items (capship or personal trading). A limited time discount for the individual trader or their station can also be achieved as Smalltalk reward (discount variable on how far & successfully you succeed with the smalltalk minigame)
As of v4b the 'hidden wares' for crafting now seem personalised to their regions, so you are far more likely to find diamonds or gems in DV stations, while OL is more likely to provide spaceflly-related wares.There is a small chance of finding anything, but it's not worth the effort of searching if you're after a specific item
Note that you will /never/ get products for crafting rare equipment: you won't gain <Authentic> Eggs or <Rare> JD components from tunnel-diving no matter how much you search!