If he is that fucking stupid that he couldnt tell that manifold was for an externally gated turbo, then surely he will be unable to find your house again and it must have just been luck last time?

Personally, I would offer him a refund, but explain that you will be posting a copy of this thread on the MLR including his name, to point out to others how stupid he is and that they must hold his hand on any purchace he makes, after all what if he bought a set of wheels and they didnt have tyres, he might just fit them straight to his car and have an accident.