used  / second-hand

I learned a "new" sign today that [my] teacher hadn't known before: apparently the local slang for "used car" is signed with an "L" sign with the index finger pointed away from you like a pistol (at about a 45 degree angle) and shaking it back and forth a few times--like prodding someone with a gun to buy a car they don't really want? The person who had learned it from Deaf friends had no explanation but swore that it was true.

Loren,
Actually, that sign is based on a sign that means "second" as in "I second that motion," or "used / second-hand"
When at a board meeting if we want to second something we will use the "L" hand pointed somewhat forward and up.  The movement is a forward/sidewards roll with a quick ending.
If you roll the sign toward a person that sign can also be used to mean, "it's your turn."
When this sign is used for cars it means "a second-hand car." 
Bill