Ah! yes, I have just looked at my CS2 and the "help" dropdown has the 'Transfer Activation' option. I surmise this is more aimed at those using another PC/Mac who already have used the 2 activations and need to for whatever reason put it a new PC. If you are buying from a reputable & traceable source who can underwrite & guarantee that they have deactivated it I would hope you will be fine but bear in mind CS3 is so new I wonder why a full copy is being sold on ~ remember if something looks too good to be true then it is likely not!
Both Amazon marketplace and e-bay are full of scammers.
I just checked one of those "used" offers. Those were academic licenses, sold at almost twice the academic price, but without checking the buyer's eligibility. If I bought one, I couldn't use it professionally.
On e-bay, Photoshop is offered at great rates (I got that tip from a graphics design website). But I won't go for it. It all is so fishy. One of those "sellers" even uses a private account, hiding his location and identity. I mailed the link to this offers to Adobe to see if this seller was reputable, and if he isn't, I will drop it, and just wait longer, do my work with Lightroom only, and buy Photoshop later at the regular price.
One has to be cost conscious, but I'm not crazy to risk getting some illegal software. Both Amazon marketplace and e-bay are infested with scammers, hiding behind the names of those well disigned portals.
After all, it may be very well possible, that among those metadata in your work the serial number of the software is also embedded. It wouldn't take much to do that (but maybe it is not legal for a software company to do that, like Apple could not embed info in their iTunes songs).