There are a bunch of semi-machined rec'r kits out there. What is expected of someone who buys and completes one of these?
You are expected to know and follow the laws that govern what you can build. In addition to the federal laws, some states have more restrictive laws.

Can a US citizen own a hand-built firearm without any gov't interference?
Yes, that’s the attraction with 80% receivers, “no paper.”

In Texas you can even build your own flame thrower without any paperwork.