I agree with Jagex, it was a good idea to remove the wildy/trade limit. There was just too much real life trading for the good of the pk/trade to be countered.
You can either specify how long you want them to keep your item, or you could have it be "until logout" and then after the logout, you would get it back :)
I had that problem too. Here's what to do. 1.) Get the pebble (by reading the quest guide) 2.) With no armour or weapons, bring the pebble to grails tomb and use it on the stone 3.) Run past the moss giant, open the chest, grab the amulet and run out again