If you have a "coherent collection" and/or anything of reasonable value, the best use of the web would be to find a few book stores that specialize in used/old books, and then contact them for info on how to list them. They may have someone on their list that's already asked for one you have.

There are a number of good dealers, but the best one for you would depend on the kind(s) of books and location. The books need not be particularly valuable, but preferably out of print or at least a little hard to find.

If you have mostly stuff that you could take to the local used book mart, then the best advice is probably to just filter them into your local waste removal stream... but I know that's really hard to do, unless you've learned from giving the children away a few times (and the kids always find their way back).

John