You are correct.
You pre 94 mags are legal.
There is nothing in the law about them being in state.
The law says they just have to be legally possessed.
Unfortunately GOAL does not have the laws accessible:
www.goal.org
But this from Giffords is the same if I am right...
Massachusetts prohibits the sale, offering for sale, transfer or possession of a large capacity feeding device unless such device was lawfully possessed on September 13, 1994.
1 Under Massachusetts law, a “large capacity feeding device” is defined as: “(i) a fixed or detachable magazine, box, drum, feed strip or similar device capable of accepting, or that can be readily converted to accept, more than ten rounds of ammunition or more than five shotgun shells; or (ii) a large capacity ammunition feeding device as defined in federal law as it appeared on September 13, 1994 (that federal statute expired on September 13, 2004). This does not include “an attached tubular device designed to accept, and capable of operating only with,.22 caliber ammunition.”
2
Massachusetts prohibits the sale, offering for sale, transfer or possession of a large capacity feeding device unless such device was lawfully possessed on September 13, 1994.
giffords.org
It just says lawfully possessed. It does not say it has to be in state. The government in Mass put that in there to create ambiguity while not going so far as to ban them. This is what Mass law does with guns.
Regarding your sales...
When selling on auction sites - ignorant people would often question my mags.
This was when I had lots of mags for sale and wanted customers. I would spend time and effort educating my customers in the hopes of making the sale. It always worked in my favor but took lots of time.
Nowadays it's not worth it. I only have a couple things for sale that are in high demand and don't need to deal with tire kickers. My stuff will sell sooner or later and I am in no rush. An informed customer is your best customer.
I have a small markup and taking 30 minutes to email back and forth takes away all my profit that I gauge at $15 per hour in effort.
Don't blame this guy for citing Giffords.
GOAL should be the first place to appear on google for this information but they are not so google shows whoever has the more complete website.