Soulbound items are always tied to individual players.
All items that drop from an Elite are soulbound to a player, If you attack and kill an Elite with another player, loot will drop for both players. Only you can pick up your own loot soulbound to you. This is a system in place to prevent players stealing good loot drops and giving you the crap loot, making the drops fair for everyone.
Soulbound items cannot be sold in the Auction House, but other items like Elite enchantment books and charms can.
Items can be unbound by using an "Unbinding scroll", This scroll can only be obtained by completing the Lv 200 The binder of worlds.