Gnu.org states that it is perfectly fine to sell your modified version of a “free software”:

Since free software is not a matter of price, a low price doesn’t make the software free, or even closer to free. So if you are redistributing copies of free software, you might as well charge a substantial fee and make some money. Redistributing free software is a good and legitimate activity; if you do it, you might as well make a profit from it.

Then they also state that it is fine not to pay for such a thing:

With free software, users don’t have to pay the distribution fee in order to use the software. They can copy the program from a friend who has a copy, or with the help of a friend who has network access.

So is it only an option for people to pay money for it? Let’s say I sold it to their friends. Is it ok that they just copy it from their friends? One had to pay but the other doesn’t? How is this fair?