I don't know what derivative means but i would assume that it would'nt be accepted to sell .sma's either.
I think BAIL said this one time in irc, that you could sell .amxx files just it wouldn't be smart because you have to give the .sma along with it.
I think that the HL license would stop you from being able to sell anything though directly through the means of "You're buying the file". Instead you can sell "The time used to make the file".
All-in-all I think its possibly legal to sell .amxx files.