Typically they are very hard to sell in the G.E. I found it's best to hold on to them and use them for rc as body tiaras are the best f2p rc xp. I've also known friends who have put tiara's in as junk when buying rares.
Since buying big bones in f2p can be VERY costly, i suggest camping at the ice giants for a while. They drop big bones 100% of the time and there isn't nearly the competition you have to face at hill giants.
Your combat level is high enough where you might want to head down into the edgeville dungeon and kill hill giants. They drop big bones which can be sold for over 400 ea on the G.E. Bank at either the G.E. or Varrock West when you have a full inventory. You might have to world hop to find an uncrowded world.