Keep an eye on Lightnetwork.com, occasionally people will sell old dongles that have are a few versions back. However, I have a feeling that you can not re-sell the education edition... that is usually the case with education based software.
If you do find something used, also be aware that
WYSIWYG/Cast does not give a crap about people that are not
current on the software. If you have an older
release and find a
bug, cast does not care and will do nothing to fix it beyond saying "buy the new version".
If you do want
WYG, I would highly suggest you bite the bullet and get a
current subscription and keep it active. Otherwise, you have no support and a fairly useless product. If you do find something used, I would not buy anything below
release 23.