Buy your book online

Have you ever buy your text books onlie? If you have done that, you must have figured out the price online is much cheaper than the textbooks in bookstore. and sometimes, you can get a new book with very low price. But the price online is not always very low, it has some fluctuation which according to supply and demand. If there are not too many a certain book, the price of this book will be higher. The prices are set by sellers according to how new(new, like new, acceptable) they think their books are. So do you think online book market is Monopolistic competition ? which means involves many firms, each having some price-setting power as a result product differentiation(cited from Comparative Economics).
And do you also think online book market is a underground economy? http://www.freerepublic.com/focus/f-news/1312260/posts
Will it affact the accuracy of GDP? I think that if we considered used books, we cannot put the income from selling the used books into GDP, because they may not be produced in a given year. However, if we imagin selling books as a job, the income from it should be people's wages which is part of GDP. So what do you think?