The problem with building and selling it would be

a) Kickstart ROM, although I think this could be solved by bundling a copy of Amiga Forever with each one sold which adds about £15 to the cost. I'd imagine giving a licence for AF would allow you to include the KS on a chip.

b) Electronics licencing - Not sure on the exacts of this, but don't electronics have to pass some safety certifications before they can be sold?

However these two are bypassed by making it a 'build it yourself' project. The end result if you have the skills really is quite impressive: