If that's the case and you know he is like that you shouldn't have taken on the responsibility of selling something only to rely on your waster of a brother to do all the work.
Some genuine honest buyer is getting shafted because of a moment of stupidity on the part of you and your brother and it isn't fair on him. He has every right to be angry.
I'm not saying he doesn't, I didn't come on here all guns blazing saying "OMG THIS GUY IS YELLING AT ME ON EBAYZ"
I came here asking out of curiosity how others would have replied to him
As you seem to have suggested, get the remaining parts sent out to him ASAP or give him a refund if you cannot do that.
A refund won't happen, because I will make sure the parts are sent to him, be interesting to see if he kicks up a fuss after all of it


