To expand your base wherever you want, build sandbags out to where you want to build a structure. They are cheap and you can build base structures/defenses adjacent to them. They can also be used to sell infantry that are close enough to them by holding the sell cursor between the sandbag and infantry. Vehicles can also be sold like this and its faster than the repair bay. Sandbags can also be used to trap the enemy AI in its own base to cut it off from supplies or from attacking you. They will not destroy it and cannot traverse over it. Chain link and Concrete walls can also be used, but sandbags are the cheapest.