According to the Unity store, Unity Indie (now known simply as Unity) is free for personal and commercial use. So you can sell a game you make with it.

If your company ends up grossing over $100,000, you will need to buy Unity Pro (though you probably won't mind at that point). From the license comparison page:

"Companies or incorporated entities that had a turnover in excess of US$100,000 in their last fiscal year must use Unity Pro."

See also: http://answers.unity3d.com/questions/6/do-i-have-to-pay-unity-if-my-game-makes-a-lot-of-money

As answered already: You can just sell it, the difficult part is how to sell it... I've been researching fees and features of payment processing and fulfillment solutions. These are my favorites so far:

Fastspring (8.9% -OR- 5.9% + $0.95, but a lot of features - watch their demo)
PayPal (1.9% - 3.4% + €0,35 - there might be charges for withdrawing the money?)
BMT Micro (9.5%)

Then there's only the problem of getting people to want to pay for the game :)

PS: Sorry, can only post one link because users new to this system only get one per post...

As mentioned above, the difficult part is selling it. But more in the way of getting people to buy it and advertising.