On the one hand, you'd get a better price by reaching a bigger audience if you could offer your fabrics online (Ebay, Easy, etc.) but it's a lot of work to photograph, measure, and advertise each piece, not to mention the actual selling and shipping. You can have a sale right from your home, but you are limited to people who live nearby and generally those people want a screaming deal.
I suspect that since you often don't get all that much money for the effort, many people just come to the conclusion that it's easier to donate.
I have had some success selling fabric, but not enough to get excited about. Hope that helps!