At first I thought 635 demos probably included too many PC demos to be reasonable, but that's actually not the concern. The concern is that many, many of these are repeats. Rare or not, asking $2000 for this is pretty brave. Maybe there really is a collector willing to drop that kind of dough on redundancy.
Or maybe the rares in this set add up to over $2000 and the seller is hoping to find a reseller who wants to take the time to resell the rares separately.
Except that it's not an auction. Leave him an offer for $50 and see what it gets you. ;^)