Use both Amazon and Ebay but you have to be selective and as far as Amazon goes you have to be patient. Books can take years to sell on there but if you have many listed you would have a steady flow of income.
Be careful not to waste lots of money of ebay paying fees up front for items which wont sell. Be very careful what you list and do some research first.
If you're selling just a few books, Amazon is probably the best place (but that doesn't prevent you from listing them on eBay).
If you're trying to run a business selling secondhand books, the only place to go is where every serious book collector goes, Abebooks. It's not cheap but it's the market leader: http://www.abebooks.co.uk/docs/Sell/