Used Car Salesman

Used Car Salesman is a Halo 3 achievement. It is awarded for destroying a vehicle with three or more enemies in it in either a ranked online playlist or in campaign. Unlocking this Achievement gives the player the Scout helmet for Spartan armor customization, it also awards the player five Gamerscore. Note that Scarabs do not count as vehicles for the purpose of this achievement, as the enemies must actually occupy a "seat", rather than merely stand aboard the vehicle. It is represented by an eight-pointed yellow star on top of an eight-pointed red star. On top of this are three white stars.

Method 1
An easy method to get the achievement is to start a campaign mission where there are enemies on prowlers, then weaken the prowler with weapons fire or frag grenades, then destroy the vehicle with a rocket launcher, spartan laser, or a plasma/spike grenade stick.

Method 2
Alternatively, you can also destroy the front turret of a phantom during a campaign level. This can cause a glitch which unlocks the achievement, as the game thinks the Phantom is destroyed when only its turret is. There are many levels that provide this opportunity: Sierra 117 (After saving Sgt. Johnson), Crow's Nest (When clearing the Hangar), Tsavo Highway (Before acquiring the second Warthog), The Ark (When ambushed by a Phantom in the Cartographer) and The Covenant (When fighting in a Hornet).

Multiplayer
The easiest way is to get it on a large level with Warthogs, M808 Scorpions, or Prowlers, like Standoff. Grab the Rocket Launcher or the Spartan Laser. Wait for the other team to grab a Warthog and destroy it. It is best to do it on a Capture the Flag gametype, since their players tend to use team vehicles.