Ebay is where I get most of my books but you have to be ebay smart or you can get some really bad shape books.
The problem is I seen alot of sellers have 10+ copies of a book but these are usually new not used.
Most sellers will combine shipping. I've gotten alot of great looking paperbacks through ebay and Amazon Market Place.
One thing I have learn is never buy anything below Very Good condition unless you want a copy in so bad of shape it's disgusting to touch. Look for sellers who post actually picture of the books not stock photos. Read their feedback. alot of time you will see books was worst then describe. Move on if you see that by more then one buyer. Buy only from merchants that are in your country. Try to buy from sellers who actually go into detail about the used condition the book is in.
Amazon Market Place you need to be more leery of. Alot of times the sellers are same ones who sell on Abebooks and you can either get rooked or receive books in really bad shape. I was once sent someone elses order. I got my money back because the seller never got back to me. I once got a book that was soaked because the seller didn't use plastic to wrap the book in and sent it in a paper mailer.
Never had an issue with Ebay sellers like Amazon sellers when it comes to books but you have to know what you are doing and who to buy from. Never buy books listed for a penny. I have and well they are usually library used books that are falling apart. These are the sellers to stay away from.