This is lifted from the Calif. DMV code: (I assume you live in Calif.)
"All Clean Air Stickers remain with the vehicle they were originally issued to and cannot be transferred to any other vehicle. If you purchase a vehicle that has a Clean Air Sticker you may transfer the sticker to your name.
If you acquire a hybrid vehicle with Clean Air Vehicle Stickers attached, and are a resident of Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, Solano or Sonoma County, you must obtain a Bay Area FasTrak account before you drive as a sole occupant in a high-occupancy vehicle (carpool or diamond) lane.
All Clean Air Vehicle stickers will expire January 1, 2011, unless the law authorizing their use is extended beyond that date.
Carpool lane use may be restricted at any time by state and federal law for all Clean Air Vehicles carrying fewer occupants than the posted minimum requirement, if their presence is determined by the California Department of Transportation to contribute to increased traffic congestion, increased travel times, decreased sustained travel speeds, or other factors affecting any carpool lane or segment of that lane.
Clean air vehicles that meet the posted minimum occupancy requirements for carpool lanes are not subject to the above restrictions."

And so - I would guess you couldn't transfer your sticker. Whoever gets your car, gets the sticker.

Or so it seems.