A trading Quack may be a vendor, trainer or tutor who teaches or sells a paticular method or system of trading the markets, but does not in fact trade for himself - and if they have traded at all may never have been a profitable trader.
Such services will be sold using exaggerated marketing with questionable or unverifiable performance, since often the methods sold will not be profitable in practice.
Origin: The use of the word quack derived from the same word used to describe one who practices fraudulent medicine, usually in order to make money or for ego gratification and power. Their services were sold to people who may have been genuinely suffering.
 See also