I truly don't think that any animal is a lost cause, even if you KNOW that they're going to die.....If nothing else, at least someone can show a mistreated animal what it feels like to be loved and have proper care, to make their last days feel like they matter. And animals who are very sick sometimes surprise you and recover beyond our wildest expectations. As my son is fond of saying, "sometimes a lot of love will cure what all the medicine in the world can't"! Loving an animal and treating them kindly is never a waste, no matter how bad off they are healthwise.
However, if anyone here is interested in trying to save this poor little guy, I certainly wouldn't advocate paying this nitwit $300 for him. He certainly shouldn't be rewarded for treating an animal like that. If someone would like to rescue this beardie, then I would simply contact this person and offer to take him and give him a good home. It they give you a hard time about it, you can simply point out that the tank and equipment that he has is worth the $300, even without the beardie, and he could still list the equipment and tank on Craig's List.
If he still won't give the beardie up, then I think maybe a call to the local Humane Society or Animal Control Officer in his area, complaining about animal neglect might be in order. Unlike us, they can just go in there and take him, if he is being neglected or mistreated, and perhaps they would be able to place him with a Rescue Agency of some sort.
People that neglect or mistreat their pets make me so angry that I'd like to just take them somewhere that's uninhabited, tie them up, and do the same thing to them, but, unfortunately one would go to jail for that. If the laws for animal neglect and mistreatment carried a mandatory prison sentence, rather than just a slap on the wrist and a fine, I think we would soon see people being more responsible and humane in the way that they treat their pets. I sometimes wonder just how long it will take for our lawmakers to figure that out!