Everyone in the gridseed groupbuy received their miners or a refund except your brother,
I don't think that really works as a defence, you're admitting a debt but saying its his [his brother's] fault you owe him money. It doesn't stop you owing him the money.
I owe him his miners but am unable to fulfil it because he is withholding his shipping address. Thus I can not provide his miners and for that reason I am auctioning them off and will send the proceeds made from the auction to him as that is the only way I see that will work in this situation.
If he bought 3 btc of miners and never received anything, isn't it simpler (and the legal solution) to refund his original payment? It doesn't matter what he bought if the transaction wasn't completed, its null and is cancelled.