Making money for building or vehicles
In order to earn money for building materials, equipment or vehicles you have to collect things and sell them to the MACV trader. Almost everything can be sold, but the really worthwhile things are gold bars (1500), silver bars (750), foolboxes (1200), closely followed by tools and metal parts.
You should buy your own vehicle from the first money earned. A simple jeep (3000) is completely sufficient! With this you are mobile, it can be locked and you can collect a lot of things in it.
Also important if you kill opponents, check their inventory! Often they have valuable things with them what is sellable.