OMG Porn ! The beasts ! Seriously, who cares. Everybody looks at porn. (well, I'll leave the 2% of those seriously disturbed enough to think that it's unclean and all that)
But I agree that the OS should be wiped and the HD reformatted. I wouldn't want the remnants of someone else lingering on my computer. Moreover, I'm anal retentive about cleanliness, so I'd likely take the time to open the iMac and vacuum it. You never know how people keep their things, at least that's how I think. If I ever bought used, I'd be really up front in asking if they have pets, if they smoke etc ..
Doug