This is actually a good question. I bet a few of us have MA compliant units that we would rather sell to folks in this state when we escape. However, there is no way to know what a fair price is. I was in a MA shop a few months back inquiring about selling a AR10 lower that I've had for a while. The owner told me that, as an FFL, he could not sell it. Another fellow in the shop overheard the conversation and went to the manufacturer's website. He saw the price online and said, 'This is what they are getting on their website. I'll offer you that...'
We ignored him, as he was butting into a conversation he was not invited to. I felt like saying, 'Yeah, you go ahead and order it online and see how that goes.'
No need to rake anyone over the coals, but, we didn't just fall off the turnip truck either...