[Kaomie]

You have to be really clear on what you do. You will be getting royalties from distributing third-party code. This is subject to licensing, period, regardless you call it "selling code" or what else. Fortunately I know of very few authors that actually include a license with their addon and if MazzleUI does not include any of those then you can do whatever you want (you will never run into the "free" as in freedom anyway since addons are plain text and are their own source, only the "free" as in costless).

You may probably be able to sell the support or sell alternate way of distribution (faster downloads, CDs or etc...). There is a very narrow frontier between selling added value to a product but keeping the core of the product freely accessible (which mean selling support or accepting donations or ...) and restricting and selling access to the product itself.