I find manufacturing flaws can be a little bit of a grey area. I purchased a piece a couple of years ago, which had a rather noticeable internal line running through the rim, which at first glance looked like damage, although on closer inspection, looked like it occurred during manufacture. I remember being a little disappointed when the item arrived, primarily as the flaw hadn't been disclosed when purchasing.
In essence, I am more than happy to purchase an item with manufacturing flaws, however, I believe the problem exists when the flaw hasn't been disclosed at the purchasing stage. (In my case above, I had also requested a condition report on the item, which also omitted the flaw!) More than happy to make an informed decision when buying a piece, but when some facts are omitted, I think that's a completely different situation.