Ebay is flooded with used books and unless you have a rare book that might be signed by an author and considered a collectors item. You could in fact make a loss for your trouble and at best break even and still have wasted a lot of your personal time.
As far as Amazon goes they are pirates with the prices they offer for used books and in my honest opinion if you have a large number of books to sell you will get a much better price from smaller online book buyback sites.
Ironically if you look to smaller sites that buyback books, you generally get a slightly better price for your pre-loved books.
In fact you may be surprised to see the prices paid for many books are quite competitive.
I sold fourteen books just recently and the average price I was quoted for each book was around $6 dollars.
I received a little under $85 for a for about ten minutes work!
But it really depends on the books that you have.
Convenience is a big plus, not having to waste time organizing ebay listings or having garage sale and wasting a whole day watching people rifle through your unwanted book collection.
Having a website buy back books you no longer want or need quickly and easily and not having to worry about poastage is a valuable convenience.