Matt does own the posters he has listed, and most of the stories you will hear about him are true (including the Warren one).
One of the more famous ones was when he was about to have the first million dollar poster (lot) sale - it included the Frankenstein teaser 1-sheet, a German Metropolis 3-sheet and a few other pieces (maybe King Kong 3-sheet? I forget but those two stick out in my head obviously). The interested purchaser was a famous actor, and at first Matt insisted he would only accept cash, after that didn't work, he finally got the guy to do something like 100 $9,000 money orders and $10,000 cash, but then kept changing the arrangements of how it could be picked up, insisting that he be able to film the transaction and promote it, etc. until the buyer finally had enough of the shenanigans and backed out.
But absolutely many of the rarest, most expensive pieces in the hobby have passed through his hands.