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We made it share-alike as any of our project because we believe in the Open-Hardware comunity as a wonderful way to make improvement in 3D Printing, we made it non-commercial because we are happy if you print our extruder and you use it as you like. If you are planning to manufacture our extruder and sell it as if it was your work we're not as happy because we spent many days designing it.
If you want to sell it make your extruder model and sell your work not the work of others :)
On the other hand, if you want to sell the models you printed with our extruder i don't think there are any restrictions.