They use phony testimonials from people, some claiming they're making thousands of dollars a month. It's highly unlikely, or most probably impossible, to make that kind of money.
They ask applicants to send a resume, as though they're genuinely interested in credentials. This may actually be a cover for identity theft, and sometimes the scammers even say their victims must undergo vetting that involves them handing over details like passwords and Social Security numbers.
They include some fine-sounding words in a privacy or ethics statement. Others, especially online, provide a disclaimer (which they assume no one will read), a fine print document that actually admits their claims about how much you can earn are just that -- claims.
Many advance payment mystery shopper scams include a package of guidelines on how to do that job and a set of forms that are supposed to be completed after the mission and returned to the company. Makes it look like the real thing.
Online sites claim their service is free, when really they are just gateways to other sites that charge for the kits and lists mentioned above.
They advertise on respectable Web sites, TV and radio stations and in reputable publications, hoping this will give them credibility, when in reality these media are unable to check them out or vouch for them.