Yes there is, it uses the received signal as a power source.
The components use the power from the received signal to power them. Not having an obvious power source does not make a circuit non-electronic. I could knock up a radio capable of receiving AM signals that does not have a battery, PSU or similar external power source but will produce a decent signal for an earphone.
Regarding the components, What you describe sounds like an RC filter which would not boost the signal. If the resistors were inductors then you could have an LC resonant circuit that would boost the signal to some degree. I suspect that the device will work, only enough to make it legal and only in some circumstances (ie close proximity to the transmitter).
I suspect that you are correct in that it is of a low enough value that people will write it off rather then return it.
As an aside the best "low value write off" scam I heard of was a 'miracle' penis enlargement pill. Men sent off the money but received a company cheque refunding their cash and a note saying that it failed FDA approval. The biggest problem was that the cheque had the company name "The Penis Enlargment Company" plastered all over it so very few men cashed them. Probably untrue but still clever.