No, actually, if they want their product back, they are obligated to pay however much is necessary to do so. They have no leverage other than that. They're spending a very tiny amount of money to repress something that would become quite popular without them.
That's actually probably completely wrong. Just because the guy got it off of eBay doesn't mean it's not actually property of Blizzard. That's like saying that just because you bought a stolen phone from some guy in an alley, the rightful owner can't demand it back.
That's a bad comparison. A stolen phone is 1) Likely reported stolen. 2) A finite resource. This is a printed CD-R that Blizzard likely didn't know existed.
It really doesn't matter. If they go to the police and say "we believe that to be stolen property, he's refusing to give it back", they're going to come and this whole argument will be rather specious.