Proper Scrapper. Achievement requires you to sell (Crush) 5 cars at Mike's Junkyard.
You can start working on this achievement from the very first time you're introduced to Mike in Chapter 2.
You'll notice a big car crusher/compactor in the junkyard close by to where Mike's office is with some cars parked just next to the machine. Break into the cars and drive them inside the crusher. Then head up the steps above the crusher and press 'X' to have the car crushed for money.
You can come back here at any point in the game to crush cars and work on the achievement as you play. Or a quick method to knock this out would be to crush the cars near the machine and then drive out of the area and come straight back. More cars will spawn back after leaving the junkyard.