Could somebody remind how to sell something to another player?

If it is an item, go into ships stores or secure stores while landed on a planet or settlement. On the right side of the inventory list for each category is a trade button.

If it is a resource, you have to be on the planet/settlement where you have the item. Or if you have the resource in ship’s hold, you have to place it into CS (commercial storage). Then enter commercial storage View – enter commercial storage – View all items On the left you will find the trade button for each resource in the list.
If you don’t know where you left the resources, go to commercial storage View – View all store contents to see every resource you own and where it is located.

While you are in the trade window, you can set the quantity to send and the price for the entire shipment. You will need to enter the exact captain name of the person you are trading to.