I have looked at this pretty Barbie dress here and tried to figure out if it is genuine Mattel or homemade. I have been unable to find a dress like it, despite doing numerous internet searches. Since I assume there is no label in it (and the style of the dress is of the era when Mattel would have labelled it) and because I can't see the snap (?) fastener on the dress (which I think would have been small square silver) my opinion is that the dress is a well-made, homemade dress. That doesn't make it worthless, by any means. It's just that an identifiable Mattel-made piece is typically worth more than an unmarked, unidentified one. So there is my two cents opinion (since even the Barbie experts, which I am not, have said nothing about the dress).
