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If you offer paid scripting service it is definitely legal as you get paid for the hours you spent working for the client or whatever. @Garr you aren't paid for your ability to work, you are paid for the effort you spent doing a specific work which is discussed beforehand with the client.

Your script will definitely include libraries and plugins that the script depends on in order to function properly, the library and plugin owners may not allow redistribution of their work and in this case you cant sell your script as you'll get paid for others' work that you used to complete your script, otherwise, write your own libraries and plugins.

So, it isn't legal to PUBLICLY sell your script, like in this forum, not sure about private payments but if someone is interested in having your script for a discussed amount of cash I feel it's safe/legal to do so as long as you are not redistributing anything, like just sending the .pwn file without including libraries or plugins of any kind.