I don't think that laptop analogy works. If someone (re)sells a faulty laptop then they may indeed not realise that laptop has a problem - you could argue they should have tested it better, but at worst they've been negligent. But the product in this case was a document written by the lawyers; they can't claim they didn't know what was in it, because they wrote it! If it really did just contain the background notes supplied by the buyer, without any extra research or substantive legal opinion, then it sounds to me that it really did con him out of that money.