Generally with Ebay pickup is free and buyer pays postage. You do kinda have to say that though - some people will try anything.
As Nikky says, just use 500grm or 3kg bags and then it's easy and it's all worked out before they buy. That's what i did even when selling and giving stuff on BB. Just use prepaid satchels, then it's fairer to everyone and easier on you because you can just go to the post office once to buy them, then they go in the box like normal mail.
I have no girl children, nor any reason for new shoes. Sigh.