Hi there,
Suthrnjewl sent me this 'cause she knows I sell books a bunch of places. :-)
I started out selling on Alibris. At the time, there was no monthly fee and they listed the books on Amazon for you. Since then, they've added a monthly fee and they no longer list the books on Amazon.
I still list on them, and make money there, but they aren't a huge portion of my sales. I also list directly on Amazon now and on eBay (and some smaller venues - biblio and valore and half but I sell VERY few books on these three).
Different stuff sells on different venues. My ASP is much higher on Alibris but I sell more volume on eBay and Amazon.
I also use a service that manages the listings on the various venues - so when something sells on Alibris, it's deleted from my listings on the other venue. It's the only way I could keep it all straight! If you'd like more info write me at ironchick at ironchickstreasures dot com.