I always had good luck just borrowing my text books from the campus library each term.
(Yes, the ones for my degree program, I bought, because I wanted to keep them. But for classes I had no long-term interest in but had to take for core requirements, the library was a great option. Just had to keep going back in every few weeks to renew the book.)
OP--the site is legit in the same way half.com and eBay.com or the "marketplace" at amazon.com are. Great advice already on checking the ISBN, edition number, and--if you have plans to resell the book--whether it's a legit copy being sold (i.e.: the cover is intact). I'm a published author and I earn ZERO dollars if someone sells my books with covers removed. It's how the distributor "marks" non-returnables, FWIW. (Hopefully, they've already been sold once by then, though, and I've received my pay for the initial purchase.)
As a college professor, I would second talking to/emailing the instructor if you can. I have a sense of how much the editions I use have changed over time, or even the degree to which I rely on the textbook!
There are some cases where I tell students the older edition is fine, and others where I can tell them it's a very bad idea!
I've never heard of it, but it looks like other members have had pretty good luck with it. I'd still be careful, because it seems very odd to me that someone could be selling a $130 book for less than $5.
I have used these sites with excellent success, I purchased an out of print book , described as like new condition. When I received it, it looked like it was brand new, it didn't appear to have been used at all. The cost .01, that's right one cent, the shipping charge was more than the cost of the book.
I didn't mean to imply that there is a quick or an easy way to make a comparison between editions, or even a slow and difficult way without having a copy of both books. However, I have known people (including myself) who have used a prior edition without problems, and I've often recommended them to students when I was teaching in community college.
But, as Heather has already stated, the first step is talking to the instructor. When I taught classes, I knew the differences between editions enough to be able to offer an opinion to the student as to whether an older edition would be useable. Not all instructors do this, but most are familiar with the edition changes enough to be able to give an informed opinion. In the entry level classes, any information I gave in lecture, was also in the book. Anything I tested on, was always in the lecture. If anything in a lecture wasn't covered by the book, I would let the class know (I'd wave my arms around to emphasize the fact - HEY this isn't in the book, so pay attention).
As I told my students, you could probably pass my tests without the book, if you come to every class and take good notes. You can probably pass my class without coming to lecture, if you read the book carefully. If you want an A, you probably should do both.
Each instructor is different, but they should know how important it is to have the book, and the most current edition of it.
Another advantage of talking to the instructor - when I was teaching, our community college had the policy that instructors would be generallly be given a couple copies of textbook to use as they wished. If the edition didn't change, and you were teaching the same class during a later semester, you still might get your two copies of the textbook, so you could accumulate extra text book copies.
Generally, I kept one for myself, if there wasn't an instructor's edition of the text, and if I had them, I'd have two put on hold in the library. One could only be read in the library and had a two hour limit on checkout (you could check it out again, if no one asked for it during that time), and another could be checked out for 24 hours.
Once I loaned a student a textbook for the entire semester, because I knew she was in a VERY bad financial situation, and I happened to have extra copies from previous semesters.
