I declined an opportunity to make money advertising Zuggy's Gold Mastery Guide, sticking to my no advertising policy. Looking at the free copy Zuggy sent me, I started to think about the sense or nonsense of guides like that. So you get a post with my thoughts on gold making and leveling guides, and Zuggy gets a free link.
The principal problem of gold making and leveling guides is that they are trying to sell you information you could get for free elsewhere. How to make gold in World of Warcraft, or how to level up quickly, is frequently discussed on various blogs, or the comments sections of the many database sites. WoW even has lots of addons which help you level (QuestHelper) or make gold on the auction house (Auctioneer). So where is the market for a guide costing around $50?
The answer is probably that the usefulness of such guides isn't in the fact that they contain secret information (they don't), but how they compile that information in one package. You could find the same information for free, but it would take you a lot of time to find it all and sift through it. Personally I prefer to experiment and find out things for myself, but that isn't necessarily the case for everybody. Some people prefer to have a clear and simple guide telling them which quests to do in which order to level up fastest, or where you can go and make gold by farming or playing the auction house. As long as the guide is up to date, I can understand somebody paying for it, even if I wouldn't. I'm not the target audience for them, people who are much less informed about WoW are.
As they often point out themselves in their advertising, gold making guides are in direct competition with gold selling companies. I haven't got a clue how much gold $50 is currently buying you in WoW, but obviously there is a choice between spending that $50 directly on that gold, or spending it on a guide that tells you how to make gold. A gold guide has the advantage that it isn't against the rules to use one, but the disadvantage that you still have some work to do after buying it to actually get that gold. And it isn't totally risk-free, you pay the money before knowing how updated the guide is. If some patch changed the usefulness of the information contained in the guide, or if it presents information that is used by too many people, which makes the farming spots and auction house strategies less profitable, you might end up paying $50 for a useless pdf file. But then, of course, if you buy gold from a website you heard about in ingame spam, you might also pay money for gold that you never receive, having fallen victim to some scammer.
On the other hand, the few guides I had the opportunity to check out without paying for them did at least have the advantage of being more useful than the "official" strategy guides you can find in print. Printing and distributing a book takes so much time, that the information contained in such a printed strategy guide on a MMORPG is almost always outdated. Also I often found printed strategy guides to contain long lists of items or recipes, which are much better covered in online databases.
In summary, while I would say that leveling and gold making guides aren't strictly needed, they can be useful for people who don't want to take the time to find the same information spread all over the internet. Like RMT you trade dollars for convenience, and you risk being scammed, but at least you don't risk getting banned for it.