well that kinda depends on what price you want for the car and how reasonable that is based on bluebook and current condition of the car. If you're hoping for full private party bluebook, AND you want to sell it fast, you'll have to put an ad somewhere. If you want significantly less than bluebook and it's a halfway decent car (wanting $300 less than bluebook for a 1984 Caprice Classic station wagon won't cut it here) then a sign in the window will probably work just fine.
If you just want to get rid of the thing, I'll take it off your hands for you
