I normally do not like to post the same information as Andy Beal but I felt this was important. Google has now updated their Gmail terms to forbid the sale of gmail invites.
Mr. Beal links to an article over at the Inquirer, a news source which is known not to be so reputable. That article links directly to the Gmail Program Policies, but then when you look, it was updated June 28th, but what was updated? Take a look at the changes here.
Based on that information, Google never allowed for the "Sell, trade, resell or otherwise exploit for any unauthorized commercial purpose or transfer any Gmail account" within its Gmail program policies.
So what is really up?