Until recently, that is. I've put my money where my mouth is and coughed up for two eBay ebooks -- both with similar advertisements, promising WoW tips, tricks and secrets. I won't advertise the exact products here, but if you're tempted by similar offerings, read on for the lowdown.
Guide A cost slightly more (exchange rates work in my favour) and simply delivered a URL. I hope the seller is confident in his buyers' trust! The disadvantage here is that the seller still controls the ebook (or, rather, website) -- if I want the same information in a few months' time, I've no guarantee it'll be there.
However, the guide itself was interesting and well written. It was clearly a single player writing about his experiences and the strategies and areas he had encountered while playing. Of course, this limits the amount of content -- there's little info on PvP, for example -- but the advice seems sound, and resonates with my experiences.
Guide B, a cheaper product, delivered a huge zipfile of PDFs. Harder to navigate and search, but less ephemeral. However, a great deal of the content -- approximately one-third of the PDFs -- concerned bots and hacking, prefaced with disclaimers about the dubiousness of such activities.
The guide content that won't get you banned was fairly comprehensive, spanning powerlevelling, professions and gold-making. Unfortunately, while its scope was large, its readability and accuracy suffered. Several factual errors and tricks that are either out-of-date or plainly false, as well as repeated contradictions, led me to distrust the guide as a whole.
In summary? Buying WoW guides from eBay is as futile as you (probably) thought. While it's quite likely they'll contain useful information, you could end up with a guide that's inaccurate and incomprehensible -- in comparison, browsing WoW-related websites is free, and there's a surprising amount of useful information on the official WoW forums.