Buyer: "Had it planned at a machine shop. Have no idea if it leaked before but it is now installed on my vehicle and must have a crack somewhere as my truck won't hold an idle."
I don't know this guy, but I don't need to, based on this single statement.
Why would anyone have an intake/exhaust manifold planed before you know whether or not it is warped?
The standard procedure is to bolt it up without a gasket and check for gaps. If you find gaps, you then separate the two manifolds at the heat box, and install a new gasket, and install it to the head with the bolts/nuts loose. Tighten up the manifold-to-head bolts (no gasket), then tighten up the heat box nuts/bolts, and then check for gaps.
He is either ignorant or a liar.
As far as the "customer is always right". This is always good business, however it is up to the seller to provide the customer with enough information so that they can be "right". This skill is what separates the men from the boys in the world of customer relations.
I hope, for your sake, the guy is just ignorant.
Carl