Do you belong to a quilt guild? Even if you don't, I would contact any local quilt guild presidents and see if you can post it for sale there (hopefully with an explanation). You may be able to put a stack of flyers on the table for their monthly meeting and/or advertise in their newsletter.
Here is the Longarm University website for machine sale ads:
http://www.longarmuniversity.com/machines_for_sale.htm
House of Hanson is another one that is supposed to be good:
http://www.houseofhanson.com/formerlyowned.html
Also, there are some groups on Yahoo that have advertising pages. Website to join groups is groups.yahoo.com (you need to create a Yahoo ID, but groups are free). Groups with the potential to place an ad would include homequiltingsystems, sewitsforsale, gammillowners, statler_sisters, and there may be others. You can search Yahoo groups for possibilities.