While there are legitimate mystery shopping companies, 99?f the opportunities you find online are scams
1 - you should never pay upfront to join any legitimate mystery shopping company and never pay monthly fees, scammers will say you need to pay a registation fee or pay a monthly fee to access assignments. ALWAYS make sure any company is a member of the MSPA
http://www.mysteryshop.org/index-na.php - they are a global trade association for mystery shopping companies and most legitmate companies are members
You can search for legitimate companies near you on their website
http://www.mysteryshop.org/searchmspa/search.php 2 - You ALWAYS spend your own money on any assignment. Mystery shopping will NEVER send you a check in advance, that's a VERY common scam that will put you thousands of dollars in debt and possibly get you arrested
http://consumerist.com/2009/07/victim-of-mystery-shopper-scam-arrested-spends-night-in-jail.html 3 - with most shops you do have to buy products but will only be reimbursed up to a certain amount, if at all. For example, you might be asked to do a mystery shop at Office Depot or Staples and you have to buy something and will be reimbursed up to $1, but there might not be anything in the store for sale at $1, so any additional money is coming out of your fee, which is generally $5-10 per shop. Or at a restaurant, they will reimburse your food bill but not your drinks, so while you are getting $40 worth of free food, you are paying your own money for any drinks. Or at hotels, they will only pay the room rate - you have to pay your own parking, food and drinks, tips or any other additional expenses - so you do end up spending money you wouldn't have spent if you didn't do the shop
4 - most companies reimburse you within 30 days of receiving your evaluation form and receipts. BUT companies look for any reason not to reimburse you. For example, if a form is not filled out correctly or you forget to tick one of the boxes, the form is considered "incomplete" and they don't have to pay you. If the envelope your evaluation and receipt is sent in is not stamped within 24 hours of your shop, many companies won't reimburse you. If the receipt you got doesn't have a date/time stamp they don't reimburse you. If your shop does not take place at the exact time of your assignment, they won't reimburse you - for example they may say you have to go Tuesday between 2-4pm. But you go at 3:30, do your shop, but then there's a huge line to check out and the receipt is stamped at 4:01pm -- you won't get reimbursed
You are never going to get rich as a mystery shopper - when you count all the time it takes you to commute to/from the shop, conduct the shop, fill out the evaluation and go to the post office to mail it, you are making much less than minimum wage. An average shop will take you 3-4 hours total and you are lucky to make $10 for that shop. Also because most companies have many more shoppers than they have jobs available, you are lucky to get 1 job every 8 weeks unless you live in a large metropolitan area like LA and will be willing to spend time in traffic driving to assignments - but you'll probably spend more on gas than you'll make on the shop