The eBay economy for Google Wave invitations has been pretty healthy
news.cnet.com
news.cnet.com
I listed them at 9:00 AM EST and they all sold in the hour between 2:30 PM and 3:30 PM. From what I can tell they went for a high price when others weren't selling because I marked them as international.
1. To sell invites would be "unGOOGlich"; that is, it may hurt Google's perceived "do no evil"/not-just-in-it-for-the-money/faux altruism brand.
2. Scarcity marketing increases demand for an already sought-after item.