Being an avid 30 carbine shooter I can tell you that not many people shoot this round unless it's from the M1 carbine or maybe a Ruger Blackhawk. With the exception of it's use in an M1 rifle, it's fairly uncommon and not too many people have a use for it. I tend to like the round and have a couple of firearms chambered for it, ironically, none are M1's. I don't hunt anything large with firearms, typically nothing larger than a prairie dog and even for prairie dogs, albeit accurate, the 30 carbine lacks distance. So it's basically a plinker and as far as plinkers go, people who plink typically like to shoot lots of ammo at a fast rate, as when fired from an M1. So, a single shot pistol barrel hardly fits that description and personally, I don't think there are too many people out there like me that actually like the round.
To be honest, I've been looking for a 10 inch or preferably 14 inch 30 carbine barrel for my Contender. The problem I have is that when I search for a used one, they guys that have them think exactly what you've been told; that they're rare and can be worth something. Typically, the interpretation of that is to try to sell the barrel (used) for the same price in which a new one can be purchased. If the barrel is in good condition you're probably not going to sell it for more than $180 to $200 IF you can find a buyer for it.
