Ok, practical advice here. For shops you "need" something that only takes photos of about 1 megapixel. If for the rest of your daily living you want something with a lot more megapixels, that is fine, but for shops your needs are miniscule compared to current technology. If you get a camera with the megapixels, you need to learn how to take your pictures using settings that will record with the lower resolutions or use your software to reduce the size of the picture file because most uploads want smaller files.
If the budget is tight, go to Craigslist.com for your area or to a garage sale because the chances are you can pick up an older Sony or Kodak in good working condition for $10 or less that does everything you need for shopping.
On the other hand, the purchase of a camera (new or used) for your mystery shopping business is tax deductible as a business expense. I picked up a little Kodak EasyShare last December that is a 12 megapixel. It is light, it fits well in my hand, I love the view screen, the pictures are crisp and it has that anti-jiggle feature so that it is hard to blur a picture. I am impressed with it. The instructions that came with it were a miserable excuse for anything, so I googled a decent manual for it on line. The software interface to my computer is pathetic, so I just use a card reader with the memory card to pull in the pictures to my computer to edit with other software. And the camera eats batteries so if I am heading out to do a bunch of shooting I make sure I have a pocket full of recharged batteries. Granted it is much less devouring of batteries when I am taking lower megapixel shots. The camera, however, is small enough that it works well for anonymous photos.
When I am doing a reveal shop I take along my old Sony Mavica. It has 1.4 megapixel capacity, is really easy on the built in rechargeable battery and is a big old "hang around your neck" kind of camera. The picture quality is certainly adequate for the jobs. It has been banged around the world, dropped and rained on and still keeps going. It can get around 250 pictures before needing to be recharged so is super for a day of reveal shops. Again, I use a card reader for the memory sticks and the old graphic software on my laptop rather than messing with the software that came with the camera.