[HamletEagle]

That's a broad question.

The license allows you to sell plugins as long as you always provide the source. You can't charge more for the sma file or distribute just the amxx file. In summary, it is legal to sell plugins as long as you provide the source.

There's also the legislation of your country. Selling anything is a form of income, and depending on the laws you may or may not have to declare it and pay taxes on it. We can't give you any advice on this point.