It's not the days on them that really matters at this point though. Over time, plastic and other oil based polymers (plastic, foam, vinyl, rubber, silicone) break down. This is the same reason rubber bands get old and crackly, shoe soles get firm, the lenses of headlights get yellow, and helmets in general should be replaced (even if they havent been in a crash) every 3-4 years. The boots are in excellent shape, but no one should pay alot for them based entirely off the fact that you could try to mold the liner, but i can garentee that it has hardened significantly and probably wont mold they way it should. An esy fix is a new liner, those are cheap ($100 give or take for intuitions).
But at this point, there will probably be some significant changes that have taken place in the plastic of the shell, making it more suceptable to cracking, creasing, and in the worst case scenario catastrophic failure of the boot sole. Over the years, I have seen many people show up in the shop on the first fairly cold day of the year with the entire toe of the boot cracked off and their toes sticking through covered only by the liner. Will this definitly happen? Maybe, maybe not. But it is a risk that a person runs by purchasing and using a 15 year old boot.
I would say $75 obo dollars for the boots. If the user gets a season out of them, then the boots are worth it at that point. If they chunk down on new liners and the boots last longer, then they got a deal and you hooked them up with it. Good on you for hooking up the NS community!
If no one buys them, offer them up in the free gear thread and hook some people up!