Google says they will take action to
stop the trading of Gmail accounts for
monetary profit. The Gmail service is by invitation only. Many users are snatching up common or trade names in the hopes to sell them off later for money & favors.

The activity is similar to the activity of registering domain names of companies or up and coming products in hopes to sale them to for a hefty sum.

Whole Websites are popping up in order to facilitate the trade and sale of the accounts including Ebay. Google has changed the Terms of Use policy, But it's not clear how they will react or enforce the rule.

Source