This seems to be a mixture of "led down the garden path" (meaning misled) and "sold up the river" (meaning "handed over to law enforcement", slang derived from the fact that Sing-Sing state prison is "up the (Hudson?) river" from New York City).
The enthusiastic young first-time buyer was led down the garden path by the unscrupulous used-car salesman.
The entire gang that pulled off the bank heist was sold up the river by the getaway driver after he was picked up and interrogated the next day.
To be humiliated by your opponent in an activity of competitive nature, or to be conned or outsmarted by something/someone.
If one was to beat another at a sport such as pool, and said person beat the other by "8-balling" him, then he could've been said to have been "sold up the garden path" by the victor.
If one was to have bought something for £400 that could normally have been purchased for a fraction of the price, he could also have been said to have been "sold up the garden path".