If you're new to selling on eBay, look at Tips for Selling Ex-Library Books.

For logistics and specifics, read a detailed account of the University of Maryland's Half.com sales efforts:

Kackley, B., Beygo, P., Miller, J., & Chawla, G. (2006). From clutter to cash: online book sales at the University of Maryland's Engineering and Physical Sciences Library (EPSL). Collection Management 31(3), 73-84.