Check Jobslinger as it will tell you who shops in your area. Unless the job was just posted that day, it will probably be taken (or else there may be a good reason that it is not-experienced mystery shoppers also know what shops to avoid signing up for from hard-won experience...so if it pays low and says EASY in the heading, you might want to think twice...But sign up for those companies that show up as having shops for your area. Continue to sign up for companies and you will find offers in your inbox. Most experienced shoppers are signed up with many many companies, not just 10 or 20. Start with CoRI and MFI. They do not pay well, but they have shops in most areas of the country and have easy to do shop forms, which are good training for a new mystery shopper to get their feet wet. You will soon graduate to ones you find more appealing, but it is good to start with the easy shops that are "forgiving" to newbie errors due to check the box type forms.
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“Lying in bed would be an altogether perfect and supreme experience if only one had a colored pencil long enough to draw on the ceiling."
~Gilbert K. Chesterton