Let me start by saying that my only experience buying from AbeBooks' site was great. I live in the US and purchased the Australian edition of The Book Thief from Manyhills Books. The price was decent, even after international shipping fees, and the book arrived over a month before the deadline, and it's in great shape. If I were allowed to rate the seller, I would give Manyhills Books 5 stars. However, AbeBooks itself gets two stars for the reason that you CAN'T rate the booksellers.
The rating system on this site is based solely on order fulfillment (how quickly it arrived, that sort of thing) and it's rated automatically. If you had any problems with the delivery or the product itself, you can't write a review to warn other customers. So while a seller may look reputable because of their 5 star rating, there's no way of knowing if they actually deserve that rating. Their books may arrive on time but come in unusable shape—or be a completely different item than what you ordered—and they'd still receive a 5 star rating from the system. The rating system is extremely deceptive, making ordering from this site very risky.
Despite my own great experience with purchasing from AbeBooks, I would not recommend using them. It's a gamble, as you could either have a great experience or a nightmarish experience, as you can tell from all the bad reviews. Unless you're willing to take the risk, avoid AbeBooks and try to find your books elsewhere.