I'm just going to throw this out there as a hypothetical. Based on your statement that "it was months before you could inspect the Amiga", is it possible that the A520 was misplaced and cracks occurred after he shipped it, but before you had a chance to inspect it?
I'll throw out a hypothetical to Wrath of Khan as well

How did the case get broken? Airey, Postal Service, your Niece, someone else at your Nieces house? If you sold something, and regardless of the story you were given, were contacted months later with issues, what would you do? Did you actually contact Airey with the issue? Was he at all open to listening/trying to sort the issues? (I'm not implying he did anything wrong or that you did, but just trying to put it all into perspective - as stories aren't usually 100% as they come across).
And to the other posters stating that things were "broken"...I understand that Airey has a s*&tload of items listed at any one time, so when something is listed as working, I read between the lines and assume that a quick test only was given. It's not likely that he has had the item personally for years and knows every intricate detail about it. Likewise he is not likely to test an item for weeks prior to listing. I suppose with the A3000 Zorro slot issue, you don't know the slots don't work until you open them and try them - I get that. And it's not likely Airey actually tested them. But likewise, if we are being 100% transparent and open to possibilities, how do you know the slots (or whatever related item) wasn't broken by you accidentally? Or broken by a faulty card that you installed? Or that you had some kind of power glitch on install/test? (Once again, not saying blame shouldn't be laid with Airey. But likewise, why should it go down in history as his stuffup when maybe it wasn't???).