Is it legal to sell an advance copy? These books are given out for free by the publisher. Neither the publisher nor the author makes any money on them, in fact the publisher invests the money to print the book then gives them away, in order to get some publicity for the book. At least the person selling this book, An Abundance of Katherines, is being honest about the fact that it is an advance copy. Where did this seller come up with the price? $27.49 is pretty steep for a free book.
I think it's more the novelty of owning a book that has mistakes in it. If you remember when Harry Potter and the Deathly Hallows came out, there were a bunch of books missing a huge chunk of pages. Those books were selling for a LOT of money on eBay, Craigslist, etc., because it's rare. That aside, though, I understand your point that selling this copy of Katherines is purely for the seller's financial gain. Also, though, if I had an extra $27.49, being a rare occurrence for a poor college student, such as myself, I'd probably buy it. John Green is a truly excellent author.