This is not directly multiboxing related, except that as a multiboxer you generally have a crapload of greens to sell at the AH. I always do it by searching for competing items withing 3 levels of the crap I'm trying to unload and then undercutting the competition. If you do the same thing this addon can save you the repetitive work of setting up each search. Just alt-Lclick on the bad item you want to sell and Undercut will do the search for you.

http://www.wowace.com/wiki/Undercut