There are lots and lots of people who demand stories with strong female characters. There are tons and tons of women (and smart men) who read SF and want to see smart, competent women represented. But yeah, the target demographic is smaller when you're talking about trans people.

I don't think only trans people are interested in reading about trans characters, actually. Look at how many people bought She's Not There, or Trans Sister Radio. Or the books of JT Leroy. There's a market out there, for sure.

I think if you had a speculative fiction novel that included a really fascinating character who just happened to be trans, you could sell it. I'm not going to write that novel, because it's not the kind of speculative fiction I write.