Ask yourself this; if you were super-intelligent and in some moths would manage to completely decompile, study and be able to recreate for instance the Windos Operating System…. Do you think the people at Microsoft would say “ah, so you learned how to do it?. Then it’s ok to recreate it as long as it does not to LOOK like our system/code” ?
In the end it will always remain a grey area and it depends on different things, but would advise anybody working in this area to register your stuff at the copyright office to prepare for any eventualities.
If someone buys code, modified a few things, then called it their own to sell… in the back of their minds, they still know they didn’t fully “create something from nothing”.
I get a kick out of creating something from nothing, that’s why I do what I do!
I agree, there is a fine line. Let’s say you don’t want to reinvent the wheel and you end up using a Tween class, a killer math class, or you go outside of Flash and number crunch in a Python script that you didn’t create. There isn’t anything wrong with that since that’s the intent of those classes and why there are licenses describing the “use” of those classes/extensions/etc.
But, what the OP is asking if you buy code, improve it, add features, and then resell it without the original author knowing?
That is a bad practice in my opinion. What if you extended an abstract class without knowing? Removed valuable event dispatches, etc? What if you just messed up the original file? That’s happened with noPonies on this site before. Someone took his classes, destroyed them, and resold them here. They were caught of course since FD has a strong Sheriff department.