You often hear about this idea (and maybe the other 4 as well, but I can't say from personal expertise) because there IS a need for a service like this and entrepreneurs know that. But I see two main problems that keep these book-swap start-ups from succeeding, and keep these ideas from going away.
First, there's a TON of variation in the way you can setup this business model. There are dozens of different ways to make your money, a handful of drastically different customers you can try and target, and endless ways to setup/code the swapping process. Not every solution will be successful; most will be unsuccessful.
Second, I don't think this idea works as a stand-alone product for a start-up because of a handful of factors. One such example: book swapping "season" (start & end of semesters) only occurs a few times a year. For a start-up with cash flow like that (or none), each "book season" is going to be swinging for the fences, after months of work, and hoping you can grab the attention of the market during the only few weeks they might care about your product.
Sorry if I was vague at times, I didn't wanna get into details.
I imagine New Zealand is a similar market as the Netherlands. Here, TradeMe.co.nz completely owns the second-hand sales market (eBay/craigslist). If you want to sell something, that's where you go because you know that's where all the buyers are. Sure it's not a targeted social app for textbooks, but who cares, you don't need it to sell a textbook!
Don't be too fast to discount this one, maybe not for the US, but as a Dutch student, this is definately something that needs to be built... Something social (friends of friends will often have the books you need) and easy. We have Amazon, Bol.com (Amazon clone), Marktplaats (craigslist/ebay crossover), but none are a perfect fit for textbooks.