Another update. So, it's still unclear whether he really sent the carts or not, but that's not what I was going to talk about.
After RobGBA got his Donkey Kong 2 cart, he, reportedly, couldn't make it work until he resetted his GBA several times. Well, it seems to me that the seller got stuck into a similar situation, so that's why he thought it was out of order... Yay.
So, the hunt is still on, considering I've missed my chance to rope it out.