I made $700 last year from selling used books online. I usually get them for $1 or less at thrift stores and resell them for $20 average.
I got one for $25 on eBay and resold on another Aussie site for $125. That one shocked me. I have found there are a few certain cook books and financial books that I have sold a few copies of, them adn self help books sell best for me.
I use
OpenDNS because I am in Australia to sell my books, but I buy my books formt he USA usually coz it is WAY cheaper then here.
So if you get to know what books sell well you can make an alright amount. That $700 of mine was after postage and everything, it was what I cleared.