My SILs sister does something like that. Before she got her full-time job she had to take what she could get. I'm not sure if it is "mystery shopper" or not. But what she does is shops around at different stores then reports back with the prices. She does it online and has to check in by a certain time on Sunday. She keeps all the receipts and such to record.
I thought it was a crazy thing to do....like who pays for what you buy? But she said she just does her regular grocery shopping and uses that to log. So I guess the only downfall is having to go to all kinds of different stores.
She is still doing this even after getting her full-time job, so she must like it. I can't imagine it pays a whole lot. I'll have to ask her. I'll see her at the end of the month.