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Do you really think if such a person has an item in his/her cart, he/she will still buy it once they see it at a warez site?

That's a kind of dismal outlook, don't you think? Not a particularly fair generalization, either.
Do you think that everybody who finds a wallet with money in it is going to keep it?
Do you think everybody goes to the warez sites?

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Lastly, it's a fallacy to believe that warez doesn't hurt vendors' sales. Think about it: pirated items were bought by someone out there...

That doesn't make any sense, and it certainly does nothing to discredit the allegation people might not have bought the stuff in the first place. Think about it: The pirated item was in fact bought at one time or another by someone, but that doesn't automatically mean that everyone who sees it would have bought it anyway.
And I'm not so sure some of the merchants themselves don't "pirate" their own stuff. If I were a merchant I might just do that, to get a product with a Read Me in it out there and some good free exposure. The Poser stuff is relatively cheap. If some warez  kiddie wants something he can't find for free he may very well buy it. So in a perverse sort of way, the pirated products are their own form of advertising.

Sort of like max, maya, and so on. I've read a whole lot of stories of how students or poor amateurs used pirated versions of software and went on to by it later. That's not the same thing, but it's along similar lines.