You can do it the following ways:
1. Sell items directly from your Hold: open the Ship Menu → go to the Hold tab → tap on the Sell button on a cell with the cargo.
2. Sell them at a station: go to a station → open the Exchange tab → tap on the Sell button (if traders at the station are interested in your cargo).
3. Leave items at the Warehouse. You have access to a special Warehouse located at the Phoenix Station. You can use it to store anything you like! Go there, open the Warehouse tab, and tap the Offload button to leave the unnecessary cargo.
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