Personally, if it were me that found it in their shop...
Obviously they didn't know the value. Perhaps they are underage. This could well be a "Mara Horror Story" that they will share in the future.
If I had found it (being unable to contact them): Obviously, it is yours now. I would sell it and send them a CE with half the MP you get for it.
Just seems fair imo. If I was that player, I'd be pretty upset about it upon getting forum access!
/been there, done that.