I don't think this is by design, but I've recognized female champions are not allowed to purchase specific armor from the shops such as the Barbrute Helm or the Travelers Cloak. At first I thought maybe it was a strength thing until a female with 18 strength also could not purchase these items. I entered the shop as a male then rotated to the female for buying the item, but the shopkeeper takes the money and the item doesn't show in her inventory. Also if a male purchases the armor or cloak and then attempts to trade it to a female the item vanishes!
So beware the shopkeepers are sexist !