You're doing a great job. Thanks.
I work for a large commercial firm which buys a lot of texts. We donate all the older titles to an international charity called BookAid International - one of my colleagues manages this project and other firms in the same industry (law) have the same sort of scheme. BookAid are at http://www.bookaid.org/cms.cgi/site/index.htm. They know the business pretty well and have been operating for years. They're UK based, but if you have old stock in the Uk or in Europe generally, you could always use them as a regional centre for redistribution.
Keep up the good work.