I never met my sister's Feng Shui master in person. But you can catch a glimpse of another psychic astrologist showing off his craft in the Philippines.
Buying a Trading system is like buying a kit to make a complete car like a 2016 Corvette, and dreamy uneducated newbies don't understand this. There is a huge amount of chart reading that is missing out of systems that are sold and unless you can read charts well, I think buying certain trading system or indicators are more dangerous to an account. I certainly don't have the skills to build a Corvette even though I can work on pre 1933 cars, it is like apples and oranges, and reading all the manuals is pointless cause you need special computers that can fine tune the computer components of a Corvette. It is a long dark road to learn to trade, but you will learn you see life in different ways and then you turn Crazy.
Some people are not fit for trading, either due to an older age or due to emotional state or even family conditions. They may prefer to develop systems and sell them. Some of these systems may work well. The majority are curve-fitted and will break down. Note that most system developers for hedge funds and investment banks do not also trade their systems as their job is to develop them and get a bonus based on performance. There is a tendency in forums to try to crash everything and everyone. There are exceptions. The argument that if a system works then it should not be sold is not logical. Trading is much more than a system. It requires an office, a setup, an account, often full time job. These conditions may not be met by some good developers. Again, I believe most systems for sale are junk. But I will not say "all".
Some EAs are sold or rented and have a free demo for you to assess it's qualities. I have had a look at hundreds of these. Some are better than others. It is not necessarily the expensive ones that produce the better results. I have found 2 that look good, at the present I am testing them after buying ( no free demo ) . One has produced only 1 trade in 2 weeks ! The other produces about 3 trades a day, so far all profitable but it is early days. So it is slow going. Don't buy Martingale or grid systems. They blow up spectacularly.