Half.com and Amazon used books are sold by individuals with varying degrees of professionalism. Some are full-time booksellers with many years of experience. Others are folks who list only a few items at a time. Each seller has a rating that has been determined by customer feedback. In addition, there may be comments, pro and con, about the sellers. Half.com and Amazon provide a common access point and manage the financial dealings, but in the end, it is the individual bookseller you are dealing with.
Abebooks is a bit different in that you deal with them and they deal with the booksellers. They tell you where the book is coming from (could be anywhere in the world) and the name of the merchant, but you deal directly with Abebooks.
I have used all three with great success. I've gotten books from all over the US, a fair number from Europe, and even one from South Africa through Abebooks.
Alibris is another good site and it functions similarly to Abebooks.
These sites offer the potential for great savings on textbooks, but here are some cautions.
Try to use the ISBN (book ID number) whenever possible. This ensures that you are getting the correct edition and cuts down on confusion when books have similar names.
If you don't have an ISBN, verify authors' names, edition, year published, and number of pages. Ask about whether any cd-roms that came with the original books are included. With workbooks, find out if their condition and if the answers have been filled in. With the textbooks, ask about any highlighting that may have been done.
Make sure that you look at the estimated shipping time and price. I believe Amazon and half.com have a uniform price depending on the method of shipping. With Abebooks, the shipping depends on where the item comes from.
Ask, too, about return policy. Abebooks has a good one, if I remember correctly. With half.com and Amazon, it will depend on the individual seller.
Hope this helps.