re: selling maxima program packages...
There are technical barriers to doing so in any "secure" way.
You could translate/compile it so that the source code was not readable.
That would not prevent someone from sending a copy to someone else.
It is unlikely that someone would pay for what amounts to a rarely-used
utility program like pw.mac.
If you put together a package that could (for example) do some important
engineering computation needed on a daily basis to actually earn money using
Maxima, there is nothing preventing you from selling it. And I think
that is just fine.
RJF