The Steps:

1. Select the SELL button in the Sidebar
2. Sell the building (left mouse button)
3. Select the Selection Cursor (right mouse button)
4. Select the building (left mouse button)
5. Press S on the keyboard

Note: This trick only works in versions 1.07 and 1.18p. It was removed from later versions.

To sell a vehicle, place it in a repair bay, then choose the $ cursor and place it over your vehicle. You'll get about the half the value back.

To find a space ship, go to the GDI level 6: Czech Republic. Build a normal base. Blow up all SAM sites. When you get the airstrike option, use it to attack the bottom-left corner. If you don' see a silver ship, attack with airstrike a little higher than you did before. You should see a silver space ship that crashed.

You can steal the enemy's harvester by capturing the refinery while the harvester is depositing money into it. This comes in handy on earlier missions when you cannot build more harvesters.

Direct the minigunners to stand in front of a sand bag fence. Click on the sell button and move the cursor over the minigunner. If the minigunner is standing close enough to the sand bag fence you will be able to sell him.

To keep NOD or GDI from entering your base, build sand bags to all entry points of your base, or farther away to keep NOD from blowing up buildings, vehicles, et cetera. This will allow you to build up your base without having Nod troops destroy your troops. In later missions GDI's orbital ion cannon, and A-10's will kill troops from time to time, and in Nod's Temple the nuclear missile will kill troops.

Build a sandbag wall next to one of your buildings. Now continue the wall until you reach the spot where you want to build. Build your new building, and then sell all the sandbags.