I didn't vote becuase it really depends on circumstance.
If I was buying a computer (which I wouldn't do on eBay personally, anyway, but...) and it had CS4 installed, I'd wonder just how legal it was. Unless the seller described himself as a professional who's just looking to update his machine I'd be a bit leery.
A friend bought a Toshiba laptop on eBay, the guys said "Oh, yeah, comes with all the CS4 software! After Effects! Photoshop! Illustrator!" and it turned out to be a pirated version.
Unless I could be absolutely certain it was genuine, which is highly, highly unlikely, I wouldn't value it any more.