To sell a building/item at your Home Station, you need to do the following:
- tap and hold the building you want to sell until it lights green,
- at the bottom of your screen you'll see a menu; choose the Purse icon

To sell an item from the storage you need to tap and hold it. In the menu that will appear choose the amount you want to sell, using Plus/Minus signs and tap Sell button.

Note that not all buildings can be sold; some of them are unique and important for the gameplay.