As a student, one of the biggest costs when starting a new quarter/semester is the splurge to get new textbooks. Students often won't buy a textbook if it's not required, at their own demise, due to the large costs of the book. A used bookstore is great, but costs are still often quite high (some places only chop 20% for a used book).
Another common alternative is to buy a used copy of the book from a friend or classmate, but that's generally a shaky alternative at best. If you buy it off them for half the cover price, if/when you resell it you'll not be able to make much money off it. What if there was a system that tracks the book's owners and divides the cost of the book amongst all of them evenly? The original purchaser could even be charged a smaller percentage than the rest, as incentive to add a book to the system.
As more people resell the book, the previous owners get paid a small amount for having participated in the system (funds deriving from the most recent sale). Ultimately, with 5 owners the cost is divided in 5 even parts (or slightly weighted as mentioned above). The differences are what each user gets paid (through a micro-payment system of some sort).
A book is great, if you actually want to read it. Most textbooks for class are relatively useless to many students after the class is over (much information can be found online or in libraries, and especially with technical books, they become out of date quickly). If the book can be passed on, and if the user has an incentive to pass it on, everyone benefits. Each user will get money from a relative stranger (as the differences are paid off) as reward for their involvement in the chain of book owners. As the book moves on, the reward decreases monotonically, but is still worth the effort of using the system. Also, by using the system, the monetary incentive will attract new users like flies and cause the system to grow.
All private user information must be accessible only to the user and the system. The system should not divulge information without the user's permission.
The book-selling transactions must somehow be authenticated by the seller/buyer to prove that they accurately reflect reality.
As the buyer will be actually paying with money, there's little incentive to cheat the system (it won't really accomplish much, besides give that money to other users).
As the system scales, a trust metric for each user must exist. This system should be similar to that of other peer-to-peer sales facilitating systems (like Ebay).