If you're close enough to a large city, Craigslist will get you quick, local results. I've often heard of people selling their fabric on Craigslist and expecting to sell it a little at a time, and then someone comes along and buys it all.
Also, the Yahoo group Sew It's for Sale is a popular place to sell sewing-related items online. My wife once bought a buttonholer for Father's Day for me on there.
Tate