According to one of the articles, he says he's since discovered that the pool table is also "a fake" (maybe it just wasn't screen used), and he's questioning the authenticity of other purchases he made too.
While I think 7 million is just a teeny bit high he could try to make a case for the value of these items increasing over time, as well as the value of all the items together as a set vs their individual values. He could also try to prove (though I think it'd be difficult!) that as a collector, his own reputation was damaged in some way by the lack of authenticity of the items and that this has harmed him financially in some way. He might also try to get money for the interest he's lost on the money he paid to Christies... there are ways to bend this to justify recouping more than the amount he paid. -Though I don't know if those costs would equal 7 million!
As for Christies selling something they believed to be legit, he's not saying they didn't know better but should have. He's saying that he believes they KNEW the item was "a fake" and knowingly defrauded him. This isn't just a case about a company that won't refund an unhappy customer his money. He has made this into a case about fraud. While I don't know the legal details pertaining to fraud, the fact that this is a fraud case, plus the amount of money that was involved, might *legally* entitle him to more than the amount of money he lost, just by virtue of the nature of the crime. Whether or not a crime was actually committed will of course be up to a jury to decide.
I guess we'll see what happens.