It depends solely on the domain name and if anyone else wants that domain name. Some domains are worth millions of dollars (e.g. sex(dot)com), others are worth nothing - even though people like to think they are. I've been trimming down my domain portfolio, declaring most worthless to me, or not worth the trouble of selling.
If you do decide to sell, services like GoDaddy make it pretty easy to put it up for 'auction'.