Speaking from the perspective if the researcher, there's nothing more frustrating to a design team than a "professional" research subject who seeks out studies for the pay and who really doesn't care about the product. The good recruiters know this and actively filter those people out.
They prefer people deeply knowledgeable and passionate about a particular subject. Those people don't really demand much compensation for the opportunity to provide feedback to designers of the things they already like. So the pay is usually pretty meager.
For those reasons, I personally wouldn't count on something like this being a reliable income source.
That said, if you see a recruiting ad on Craigslist that interests you, go for it.