No legitimate seller is likely to give you the serial number before you buy it. That's called "giving away the farm" because the serial number is the key to everything.
You are advised not to try to be suckered into a too-good-to-be-true deal, because sooner or later such a transaction will leave you out in the cold with no working Photoshop. Adobe actively disables Photoshop from running with illegitimate serial numbers. Illegitimate sellers are out there, and they know how to make their stuff look very good to you, and also how to be long gone by the time you realize you've been scammed.
Think about it... No eBayer can legitimately discount Photoshop deeply - Adobe just doesn't discount it that deeply to them! Why should they? They're getting full retail price from their online site!
Are you a student? Do you know that there's a very good student discount directly from Adobe?
-Noel
