Apparently, samurai is not quite the right title for these women warriors. They were known as onna-bugeisha. They were trained with weapons to defend their households and their honour in times of war but many did fight alongside men in battle.
Japanese history records several very significant onna-bugeisha, including Empress Jingu and Tomoe Gozen. There is plenty to read about these warriors if you're interested in more.
This armour was designed for a small women of less than five feet. The mask was carved with a moustache and snarling teeth to disguise the fact that the wearer was a woman. It is made of copper, iron and leather. It's different from male samurai armour in that it is designed more like kimono so a woman could dress herself, preserving her modesty. The armour is expected to fetch $30,000-$40,000.
The auction also includes several examples of male samurai armour. The pieces are beautiful and terrifying! I wouldn't want to face any of them in battle!