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.
I don't have his money anymore it was sent to the manufacturer for his units. All I have are his units. The transaction to the manufacturer was completed and his units have been sitting here for months waiting for him to provide his postage details.
All of that is irrelevent unfortunately. Having 10,000 of sales to consumers myself, the seller loses out in almost all situations due to consumer law - including this one.
All that is going to come from this is he is going to send you a statement of debt, you're going to dispute it on the grounds that he didn't give you a shipment address, its going to go to small claims court, there will be a judgement in his favour and you'll have wasted even more time AND the money.