If you build one and decide to get rid of it a year later, you're not going to (currently) get in trouble. A FFL can always enter it into their book as having no serial, but some (many?) FFLs might balk at it. Private sales in many states will be easier.
This can of course change come January 2017.
By the way, you can always buy a stripped lower, replace your 80% lower with it and sell it without issue.
Jeff
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