Usually they are scams, but not always. There may be a reason why you have to pay to work. It happens in the off-line world just as in the on-line businesses. My friend works as a manager in a grocery store in our city. The store has a policy that all employees must wear white, button-up dress shirts, solid slacks, and solid, dark-colored shoes. If you are not wearing their dress-code, they won't allow you to work. But they don't pay for your clothes; you have to purchase the clothes in order to work for them. Similarly, you may pay for education or training in order to get into a job you desire. People spend thousands of dollars going to college to get a good job without batting an eye, but charge them $500 on-line training fee and they're screaming "SCAM!" People is sales need to spend money on products to sell and / or on advertising. Manufacturers also spend money on raw-goods.
When you have to spend money for an on-line job, determine why you need to spend that money first before declaring it a scam. Perhaps as a marketing company, they have spent their own time and resources to develop a program that allows you to advertise and sell their product with very little effort on your end. A few very legit companies require you to purchase their "Affiliate Package" that includes samples of the products they produce, you own web-portal that you can use to collect orders, advertising pamphlets, posters, etc pre-made for you, etc. before you are able to sell their product. Perhaps they have specialized training that other companies they contract with require before hiring you on. There's an online company that places people to work as help-desk personnel that are able to accept calls and work directly out of your house. However, they require you to take a basic Help-desk class before they will place you with another company, and sometimes those other companies will require even more intense training into their specific industry.
Although a site asking for money is usually a good flag, you should do a little homework and research into the company to see exactly why you need to pay money to work.