Higher Education. Existing institutions are selling inefficient and grossly overpriced products in an environment in which objective evaluation is hard to obtain and the status quo fears innovation. If you could crack that nut, it would be massive.
There is another bigger one: College Book Industry.
The current system is pretty inefficient. This probably differs from country to country. The current system here works like this:
The universities give lists of subjects and prescribed textbooks to brick-and-mortar bookstores. They then buy the books and sell them at a huge mark-up.
Students generally do not order online because of the time delay/import BS and many do not have a fixed postal address. Secondhand book sales are a bit of mess because it is not centralised, etc...
There are possibly quite a few ideas on how to make this system more effective (esp. in countries in which the internet penetration is not that high but the cellphone penetration is).
I used half.com all the way through college (ISBN makes it quick), but it would be much easier if the chosen textbooks were announced earlier (like you point out). Most of the time I was without a book for the first week to 10 days.
The lead time issue could be fixed simply by changing University policy, requiring professors to disclose required textbooks X weeks in advance of the first day of class.
One loophole I found near the end of undergrad was that the campus bookstore's website had the required books posted for each class well over a month ahead of time (so you could buy them online and swing by to pick them up). I just looked up all my classes there, copied down the ISBNs and hit up half.com or Amazon. Of course, this breaks down with last minute changes or additions made by professors.
Ouch. There is a huge degree of truth in that. Situations differ however and I know of at least a few local book exchanges that are making a living (i.e. in operation 2+ years).
I do however not know of one central/large one. There are several reasons why in some cases it succeeds and in others not. In poorer regions/countries people would look for more ways to save money (the pressure isn't really on an Ivy League guy to buy a $50 book instead of a $100 one when his tuition is $40,000. In countries where students live on less than $3000 a year it is).
A solid business idea I have seen is a chinese copy center - you hand in a book and the next day get your book and copy for about $20 (this business is in operation more than 7 years without any uhm... legal problems).
