MRSMINIVER isn't a "Mrs." Nor even a she.
The consensus opinion of many people who collect Star Wars material both on and off the forums is that MRSMINIVER is Matt Shapiro who apparently faked his death a while back, only to reappear. I don't know the details during that period, as it preceded my foray into poster collecting.
Back when I was young, fresh and green (unlike now where I'm old, wilted and grey), before I discovered the various poster forums, I contacted MRSMINIVER, who never answered my questions except for one vague response. "She" was very secretive and peculiar.
On a rotating basis, almost like clockwork, MrsMiniver/Matt Shapiro will post this collection up on eBay with an outrageous amount. Some speculate that it's just to draw attention without any intention to sell and some believe it's meant to catch the eye of some wealthy person who wants a very rare, extensive collection at any price. I liken the seasonal appearance of MrsMiniver to the carnival coming to town for a bit of amusement.
And I suspect that Matt Shapiro lurks on the poster forums and pops up using interestingly egomaniacal IDs.
Jeannie