From Halopedia, the Halo wiki
Trading Down is an achievement in Halo 3: ODST. It is awarded for trading any Weapon with another character. It is worth 5 Gamerscore.
It is represented by a Plasma Pistol and a Rocket Launcher, with a green arrow circling around them.
The achievement Swap Meet in Halo: Reach is unlocked the same way, and uses almost the same image - only with blue arrows, and a Designated Marksman Rifle instead of the Rocket Launcher.