Wanna buy a duck?
Starts with one person asking the "Wanna buy a..." question to somebody. Then they respond (as in example 1), and once you reach the end, the person who was asked asks someone else.
EXAMPLE ONE
Person 1: Wanna buy a duck?
Person 2: A what?
Person 1: A duck!
Person 2: Does it quack?
Person 1: Of course it quacks!
Person 2: Lemme hear it quack.
Person 1: QUACK!
EXAMPLE TWO
Person 2: Wanna buy a duck?
Person 3: A what?
Person 2: A what?
Person 1: A duck!
Person 2: A duck!
Person 3: Does it quack?
Person 2: Does it quack?
Person 1: Of course it quacks.
Person 2: Of course it quacks.
Person 3: Lemme hear it quack.
Person 2: Lemme hear it quack.
Person 1: QUACK
Person 2: QUACK
Person 3: (to another person) Wanna buy a duck?
Other versions:
Wanna buy a duck? (above)
Wanna buy an orgy? (A what, an orgy, does it moan, of course it moans, lemme hear it moan, *insert moan here*)
Wanna buy a dietz? (a what, a dietz, does it suck, of course it sucks, lemme hear it suck, tunaktunaktun tunaktunaktun tunaktunaktun dahdahdah)
Wanna buy a Rosie? (a what, a rosie, is she lost, of course shes lost, lemme hear her be lost, rosssieeeeeee)
