You seem to be equating airline miles with amex points. Even if the airlines don’t allow you to sell miles, what is stopping you from transferring your amex points into someone else’s airline account? You aren’t selling airline miles, you are selling points that happen to turn into miles. (Though if airlines have rules against this I assume amex does as well).
That being said, it seems to me that a lot of people are doing this business nowadays. The airlines aren’t dumb, they most probably know that it is going on and are allowing it to happen. From a technical standpoint it shouldn’t be that hard for them to detect it with all the technology nowadays. If they wanted to stop it, they most probably would.