I picked up a bunch of postcards on her & her family several months ago. The seller had other copies that they sold (for only about $25/ea tops). They were bought up and some resold on ebay for over $100 some of them--including ones of her with her firstborn and her parents. I'm glad I got mine when I did. They did the same with some of the cards I'd gotten of Sandra and her family too. The original seller doesn't have the same great stock that they used to--they had a lot on the Hesses and Greek princesses once upon a time. I think the secondary seller snapped up their stock for resale.
In some of the photos, Marie K was all dolled up in jewelry but just in a plain old room. It was quite a dichotomy--almost like dress up.
There doesn't seem to be much written about Marie. The bulk of what I've read of her comes from Marlene's book Queen Victoria's Descendants and also Art's book The Grand Duchesses. Neither one was very lengthy. She had no real position--unlike her sister Kira or brother Vladimir both of whom were claimants to the thrones of Germany and Russia respectively. Even her children made more interesting marriages.
She married young, popped out a bunch of children, was widowed and died fairly young. I think there was a story about one or more of her children not being her husband's? I think that was in QVD but I don't have my copy of the book.