Why can't you force him to take it back? He didn't properly advertise what he was selling, so let him eat the shipping bill. Why should you? You paid but received something that was not what you expected. So, after spending money on a ready to drop in, in good condition rear you now have to have it rebuilt, source for another housing, have it modified..blablabla..... sounds to me like you would have been cheaper off buying the parts and having it sourced out from the get go.
I think the guy who sold it bought it cheap somewhere (ebay?) and then decided to dump it. If he had actually ran it himself he would have noticed the broken spider and the holes.
See that a lot, especially "over there". So called "racers" who buy stuff on ebay and then start whining that their top dollar part went kaput, yet forgot to mention they got it off the bay for next to nothing or buy stuff (like wheels) cheap, say they are new (with 20y/o VW tires on them) and then unload it on some other poor soul claiming they were brand new and so on. Bottom line, there's a bunch of scumbags out there.