This probably sounds like a dumb question but why can't someone trade based on just the sma or ema? I feel like if you buy when it's under and sell when it's above you always come out on top and it seems to work very well whenever I paper trade(only in college so i'm poor). I know that stuff in the news/earnings could screw this up but I feel like i'm missing something.
Correct, as Bob says, what ever system works.
But keep in mind, if all we had to do was use a SMA to use as a signal to buy and sell to make money, we would all be billionaires by now.