As TD said, it's all a matter of personal opinion.
If the guy wants to sell something on eBay, let him do it. A professional scammer would be trying to sell digital cameras / laptops etc and get more money than $12.95. And it's not as if he won't send you the CD or whatever if you buy it.
Fair enough you can find this information for free, and fair enough I've written a fair bit of it on the XLink site for free. He's just selling a guide and I see plenty of computer guide books at the bookstore that retail for 2 - 3 times as much as what he sells this CD for, and I can find everything plus much much more on the Internet for free...
I do have one objection to the item. Perhaps make it implicitly clear that you are intending on playing games through their system-link option, and not XBox Live.
I remember buying the Flash 4.0 Bible ages ago. Worst $AU48 I've ever spent. I found out more stuff on my own and through the macromedia flash newsgroup than in the book itself. I gave it to my friend who was just getting started with DTP and he wanted to get into it and he loved the book. He wasn't prepared to go around hunting tutorials from all over the internet, sign up to the MM flash newsgroup adn sift through 30000+ posts to find what he wanted etc etc...
So, if you don't want it, don't buy it. As someone who's been scammed of $AU1200 for a laptop that never came, I think those sort of people should be shot.
My 2 cents.