That sucks. As a possible defense though... maybe he's never even
played it and got it in another trade/auction. Some people have
literally hundreds of unplayed games on their shelves that they have
picked up in great deals and simply because they can't resist buying
them... it's easy to get carried away collecting games even when you
don't play most of them.
Personally, years ago I sold a copy of War of the Ring that I had
acquired in a Math Trade. I even had it all out and looked through it
a bit (not thoroughly enough I guess)... then decided it wouldn't get
played often enough at the time.. so sold it later. When the new buyer
contacted me it turns out there was a single piece missing. I said I'm
really sorry but I'd never played it so I didn't realize the piece
wasn't there. The game was pristine and obviously never played by
whomever I got it from as well... so my thoughts are that the piece
was missing since new. He simply contacted the publisher and they sent
him the missing piece.. all ended well.
I know that's probably different in your case as several pieces are
missing, but if you can't get a satisfactory resolution... try the
geek and see if others may have spare parts for trade/sale and also
contact the publisher (if the game is still being made) to see if they
may be able to help out for a small fee. Many have outstanding
customer service and will be happy to help out.