Step 1:
Buy unregistered cars in "used car dealer"-Shop

Step 2:
Dismantle all parts and sell them

Step 3:
Go to "used car dealer"-Shop and sell the frame

Step 4:
If u revisit the Shop, he may offer you the old frame
If not, revisit it again
Every time you visit the "used car dealer"-Shop, they change their offers

Step 5:
Buy the old frame, revisit the Shop, sell the frame

Example:
2002 FD3S
Buy Frame: 1,578,151







Sell Frame: 1,670,984







Profit: 92,833

or use the cheatcode "begformoney" in garage