If I live in Minnesota, and am selling a home built ar15, can I SELL it to a MA licensed FFL dealer? Does having an FFL in MA exempt you from the ban? I understand he couldn't turn around and sell it in the state. What about 30-rd magazines?
I've searched and can't find any info on this, just stuff about transfers through an FFL, nothing about what FFL holders can and can't do themselves personally.

MA doesn’t really recognize FFL (federal license) in terms of selling to MA residents. While the FFL is required you also need a MA dealers license issued by the state.

That MA dealers license is what exempts the dealer to possess “assault weapons” and “high capacity magazines”.
I believe it’s MGL 140 section 121 (I think, I’ll try to check later but here’s a link to read through the sections).
MA General Law Sections

The dealer can sell to LE but not civilians.

As a side note and this is my personal belief…

A MA dealer is exempt from the AWB and mag capacity restrictions.

They can sell standard capacity magazines and post ban ‘94 “assault weapons” to LE only and not citizens.

If the rifle is in compliance and not a “assault weapon” per the actual written 1994 federal and state laws then a dealer can lawfully transfer it to a civilian.

The issue with above is most dealers don’t want to risk their state issued dealer license by going against the AG’s press release. Even though there’s no law prohibiting it, just feelings.

So to cover my own ass, I'd need to get a copy of this guys FFL AND his dealer license?
Or does the fact that I don't live in MA mean just having his FFL is fine and I'm covered federally and it's up to him to follow the rest of the laws there regarding what he does with it once it's in his possession?

Having paperwork to cover yourself is never a bad thing. But you have no responsibility of what happens in MA.

Send it to the FFL and you have done your part. The rest is up to the MA FFL/dealer.