Simple, because "why can't they sell it? Who cares if it's an artifact? That should only make it MORE valuable, not less. We are going to sell it and if that means going halfway across the planet, so be it."
Guys how have been playing for 35 years you aren't used to hearing the word "no".
Sounds like a pain in the *ss. For being so annoying, I would teach them a lesson: Let them sell it in a bigger city, have the Technic find it out and let them be the buyer for an astronomical amount of gold, just to be caught and questioned by the League. Strip them of all their gold and equipment and send them on a boat towards the south and let them escape in Hajoth Hakdos or something like that.
I mean, if they want to blatantly ignore GM's advice, than make them feel the consequences.
Heh, funny you mention that as "GM Advice" is seen as "railroading them"! (this actually happened just last night! :) )
It's all cool, this was more of a general "what do you do with an artifact that can't be used" question.
In all my years of gaming 99% of the time when someone finds an artifact they will use it/keep it for the party. This is one of the only times I can remember -- pretty much ever -- where there was an artifact in the adventure that couldn't be used by the party (or in the case of the Rod of Seven parts they couldn't get rid of that fast enough once demons started to hunt then down!).
Hence, I can expect my PCs will try to sell it since they can't use it. Just curious to see how others might have handled this situation.
Any my PCs aren't as bad as I make them out (most of the time). They do amazing things and keep me on my toes ... I will say that.