I do have to personally stick up for the "Sell me on this" threads, because as Crothian said, not everybody has a local bookstore where they can sit or stand and skim the book to determine if the contents are useful to them. Many people are forced due to geographic location to mail-order what appeals to them, often with no or restricted return policy.
Also, take into account the fact that rules-heavy books are often very difficult to determine whether said rules are balanced in context to your game; someone who has already bought and read the book can tell you what pitfalls there are in the rules, or if perhaps you hastily read a rule while skimming the book, and misjudged a feat, or power, or spell, as being more powerful than it really was.