www.answers.com/topic/mountebank
n.
A hawker of quack medicines who attracts customers with stories, jokes, or tricks.
A flamboyant charlatan.

http://dictionary.die.net/mountebank

Mountebank \Mount"e*bank\, n. [It. montimbanco, montambanco;
montare to mount + in in, upon + banco bench. See Mount,
and 4th Bank.]
1. One who mounts a bench or stage in the market or other
public place, boasts of his skill in curing diseases, and
vends medicines which he pretends are infalliable
remedies; a quack doctor.

Such is the weakness and easy credulity of men, that
a mountebank . . . is preferred before an able
physician. --Whitlock.


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Note added at 3 hrs (2007-12-30 08:03:49 GMT)
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you've also got your famous "snake oil peddler".