Get permission from the user that created the Tweet if you want to make their Tweet into a commercial good or product, like using a Tweet on a tshirt or a poster or making a book based on someones Tweets.

Spam guidelines are the same and can be found here. The Tweet ownership stuff has already received some criticism for further clouding an already unclear subject. If Twitter can do what they want with our tweets, including reproduction for their own financial gain, what do we actually own? asks Sha Bennett at Twittercism.