I see a lot of reviews bashing on abebooks. Most of the complaints (from what i can tell) have come after ordering a book from outside of the US. In my case, I ordered a book from Better World Books, based in Indiana. It arrived within the estimated timeframe, was in the described condition, and left me completely satisfied. I wish there was a way to review the sellers on the website itself, but it seems to not be possible, currently. I do advise anybody who wishes to purchase from the website to do their research about the seller that offers the book they are interested in purchasing. I'm so glad to have found this website. The book i purchased was in Russian, and published in the early 2000's, not even available in the most common russian books retailers. Plus the prices are amazing.
Edit 2019: Since writing this review i have purchased about 10 or more books using abebooks marketplace, from a variety of sellers. The only time something went wrong was when a seller sent me a book they had mislabeled, which was not the book i had ordered. They issued me a refund after contacting them directly. No other issues, deliveries always within estimated time frame. Note: the books i have purchased have been mostly fiction (about $3-$5 per book) and 2 textbooks ($70-$100)