Achievement links are only valid on the realm they came from, item links are only valid on realms on which that item is 'out there'. All links are only valid within WoW, so it makes sense that they can't be sent if this is a restriction on battle.net
I know about the achievement and item links. The only problem is, before this whole "battle.net" friend thing was put in, I could link achieves, items, and quests to people through WIM, but now, with the release of WIM 3.3.6, I can link items to them on my realm, but not achieves/quests.