This is not an absolute - there are of course exceptions and boundaries. Which require lawyers to explore.
So no - selling a T-Shirt with Ms. Spears would probably invite legal action from her company.
As for the Seinfeld characters - you should probably simply contact the show`s publicity department. There is a fine line between the actor`s rights to their image and the show`s right`s - which Paramount discovered when the actors portraying Norm and Cliff sued over robot likenesses. But in other instances - for example Star Trek - fan productions have been tolerated as "fair use" as long as they make no attempt at a profit and do not directly use the image of actors from the show without permission and do no harm to the value of the intellectual property.
Oh, forget the concept of "fair use." Fair use is a defense, not a license - which means it only has meaning in court. If your using "fair use" in relation to your web site, you are probably already in the deep end of the pool - please review the Electronic Frontier Foundation link below. As a web property, your content falls under the Digital Millennium Copyright Act (DMCA) and a simple letter to your ISP can get your site taken down faster than disk crash.
In the end, the old adage of "better to ask forgiveness than to ask permission" rarely applies in the world of intellectual property.
Web Architect / Managing Partner
SiteRiver: Web Applications Intelligence
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