From Traderpedia

Definition: A derogatory term used for any vendor who sells trading systems, methods, advisory services or similar with exaggerated marketing but questionable or unverifiable performance.

Unfortunately, most "trading system" vendors are quacks who's only profitable system is the system they use of selling "systems" to unwitting traders.

The origin of the description "snake oil" comes from a term used for any one of a multitude of medicines, potions, substances or mixtures sold by a traveling doctors for which extravagant curative and restorative claims were made. Snake oil was a type of quack medicine widely available in the 19th Century United States.