Run, don't walk, away from this "deal"....
Here's an excerpt from another post I made about my "deal" on CS3:
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As an aside, I just learned the hard way to beware of eBay Photoshop deals that are "too good". Last summer I bought a package with a virgin copy of CS and a sealed box CS3 Upgrade for my PC. The CS disc came with no box or manual. I paid about half the retail price for CS3. eBay seller had rave reviews. Everything installed fine and activation and registration went smoothly with Adobe's server. I've been a happy camper for the last 10 months. Today I called Adobe to cross-grade my copy of Photoshop CS3 from PC platform to Mac. They asked my CS3 serial which rightly showed it was an upgrade version. They then asked for my original product serial, which I provided from the CS disc. To my sad surprise, the CS disc was counterfeit with a bogus serial. Even though their server activated my CS3 it turns out now that I don't have a legitimate version and obviously can't cross-grade it to Mac. They of course offered to sell me the full version for the Mac. I almost went ahead and ordered, but decided to see if I had any other options. I checked eBay for the seller I bought from and found he disappeared last September with several folks complaining of being ripped off. That's my only bad eBay experience.
To my pleasant surprise though, I was rummaging through offers that came with my new Wacom tablet I bought a few weeks ago and found it was bundled with PC and Mac versions of Photoshop Elements, which can be upgraded to CS3 with a discount. Looks like I'll go this route.