This thread might help:
https://forums.frontier.co.uk/showthread.php/409098-Elite-Dangerous-Beyond-the-features-of-3-0
From it:
Materials Trader
We’ve also added a new contact called a Materials Trade, providing players the ability to hand in materials in exchange for others at set exchange rates based on grade and material type.
This will allow players to trade for rarer items or items obtained by gameplay they do not want to take part in (E.G. killing trade ships) as well as converting stockpiles of unwanted materials to ones that the player needs.
This is not intended to replace material gathering completely, but be a useful tool to help shorten the gameplay loops around gathering materials, making engineering more accessible.
• Allow players to trade one material of another in a single trade.
• Only one trade at a time can be completed.
• Material traders operate on a true barter system. There are no credits involved.
• Traders are split into types – Raw, Manufactured and Encoded.
• Each trader type only trades in their type of materials and can be found in different economy types. Locations material traders can be found are:
- Raw Materials Trader: found at extraction and refinery economies, only trades in raw material found on planet surfaces and planetary rings.
- Manufactured Materials Trader: Found at industrial economies, only trades in manufactured materials.
- Encoded Materials Trader: Found at High Tech and Military economies, only trades in encoded materials.
- Systems also need to be mid to high security systems and have populations between 1,000,000 and 22,000,000.
• Materials Traders will not show up in anarchy systems.
• Materials Traders will not show up at stations that are currently controlled by a criminal faction.
• Materials Traders will be unavailable when a station is damaged, under repair or put on lockdown.
• Horizons is required to use Material traders.
• Material traders can be found using the new map view configuration ‘By Service’ option. This has a 40ly range and only shows traders that you have discovered.
• Material traders will also be shown on the system map in the station services list.
• Material traders will appear in the Human Bubble, Colonia and Pleiades regions at economies types listed above.