I like .54 inlines in part because they allow for firing .50 conicals with sabots.
I think that they have a dedicated following, so I wouldn't say that no one buys them. If someone were going to hunt some of the largest game animals on the continent then I think that they would like to use a .54.
Folks are still buying the Lyman .54 Plains pistol and .54 rifles, especially with a round ball twist.
Maybe .54's are not as popular as they were, but folks still scoff up the .56 Renegade smoothbores.
Another popular model, the Remington 700ML was made in .54 too, and they may be worth more than a .50.
pluckit recently bought a Traditions .54 Deerhunter kit. It shoots saboted bullets really well and he seems to like it a lot.