Used Car Salesman

Used Car Salesman is a Halo 3 achievement. It was originally awarded for destroying a vehicle with three or more enemies in it in either a ranked online playlist or in campaign. Unlocking this achievement unlocks the Scout helmet for armor customization, and awards the 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.

Following the shutdown of Xbox 360 Halo online services on January 13, 2022, this achievement is no longer attainable in online multiplayer, and therefore can only be unlocked in the campaign.

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 was on a large level with Warthogs, M808 Scorpions, or Prowlers, like Standoff. Capture the Flag was one of the best gametypes, since their players tended to use team vehicles, with the flag carrier often sitting in the passenger seat. In other gametypes, players were more likely to drive with only a gunner and nobody in the passenger seat.