First, find a way to come up with $3,000,000.

Next, find a friend to do this with.

Now, go to the Classic car dealer in the bottom-right hand corner of the island.

There is an Aston Martin Zagato DB4 for $3 million- buy this and go to the "trade" center in your house.

Have your friend (or you) put it up for sale for $50,000 (I highly suggest that price) and have him/her go to their Sales History and get ready to remove the DB4 from the online sale.

After this, the person buying it, goes to where it says "Buy this vehicle Yes/No" and ask the seller if he's ready.

Press yes to buy it and when it pops up saying you have a new car in your garage tell the seller to remove it you should both have a DB4 Zagato.

Repeat this as many times as you wish and sell them (worth 1.5 million each.)