will4me can probably answer most of those questions as far as the build.
I've heard conflicting opinions on if you can sell, technically you can sell it if you did not build it with that intent but some state law conflicts with that.
Also technically you don't have to serialize it (or them) if you don't sell them but lots of people suggest you do just in case of theft or if some LEO isn't familiar with 80% receivers.
At least one company has the option of custom engraving on their 80% lowers when you order them.
If you're serious, call the BATF and check, not a conversation you want to have after the fact.