what's legally wrong with creating and selling for profit a work that is inspired by a copyrighted work, as long as the work itself does not include copyrighted images, characters, or text.

Well, there's trademarking too, which covers a lot of logos and images, and trademarking works in the "it looks kinda like" fuzzy areas where copyright law does not.

Here's an illustration of trademarking: say you want to put out a cheap, abbreviated guide to the Chicago Manual of Style. It's not the actual manual, just a quickie guide to how to use it in your classes. That's perfectly legal: you're not violating copyright law if you write it all yourself and make up all your own examples and nowhere claim to be the Real Actual Chicago manual.

But if you put out this booklet with an orange cover, the Chicago people might sue. (That shade of orange, in the context of The Chicago Manual of Style, is trademarked, I am pretty sure.) An orange booklet with "Chicago manual" on the cover could be confused with the real McCoy, and might interfere with the real McCoy's ability to make a profit. Never mind that the real McCoy is 1000 pages and your little booklet is 100 pages. If the real McCoy people someday want to put out their own little mini-Chicago booklet, which they have every right to do, your published booklet with an orange cover could be confused with theirs.

So publish your booklet with a green cover, that can't be confused with the real McCoy in any way, and you'll be safer from lawsuit.

Similarly, these t-shirts sound like they're virtually indistinguishable from what Universal might put out, and if Universal has trademarked the Serenity logo and the characters' names, then the confusingness alone is legal basis enough to inspire legal action.

I'm looking at a design that has the ship in "let's moon 'em" position. In a corner it has a symbol (something more like @ than ©, but not quite either) followed by "2005 11th Hour."

They don't hold any trademarks on Serenity stuff, for info. (I did a search on the Trademark Electronic Search System).

However, they do sell licenses for these things, and the license holders are within their rights to sue Universal if they allow, for example, shirts which should come under the official license to be produced.

Let's put it this way. If she had put out that shirt, without Serenity the movie existing, then no one would have bought it. The interest that was created in her materials came from the corporately created and owned film in question.

She's a professional artist by trade, and makes money from shirts not related to this fandom, I believe.

Fewer people would have bought it? I'm assuming that the Serenity fanbase is larger than her shirt clientele?

Well, of course fewer people would have bought it. I don't think -- just my opinion -- she would contest that the designs were aimed at Firefly/Serenity fans. However, people on this very website wrote in a book called "Finding Serenity". Should Universal retroactively invoice those folk?

Invoice them for what?

Writing about works owned by Universal without a license to do so, and profiting from it.

Writing about your own personal experience about the work and writing a work in that universe with those characters are different things.