iOS Projects: Can I help clients realize what they're actually asking for?

I do lots of freelance iOS work, and I've noticed that people approach me with "Ideas" that they would like implemented, but they are not fleshed out at all. For example, I get a request for a "textbook sales app which acts as a marketplace for students to buy and sell textbooks", with no business plan or specs whatsoever. How do I encourage more thought out plans, and what tools do I point the prospective clients to? in terms of user interaction/ stories/ design?