I always had my eye on Destiny but I didn't want to pay 80 dollars plus tax for a game I might not like after all. But since the original Destiny game is like 19.00 PRE OWNED I'm thinking of getting it. But is Taken King like Hardline for Battlefield? or is it just DLC? Should I not buy Destiny because Taken king is out? I'm confused. Am I missing anything if I dont get taken king? Is taken king just a campaign sequel? I just want to play Destiny for its online mulitplayer, I will try out the campaign, I dont care for extra campaign. Please answer all these questions! Thanks :D        

If you are in the US, you should just get the taken king legendary edition on amazon now for $40, it has the original Destiny that is for $20, plus to two extension that is for $35, plus the Taken DLC itself that is another $40. So buying destiny legendary edition for $40 get you the full game plus all DLC out at the moment.
So if you are buying destiny I would recommend the $40 Legendary edition currently on amazon. Don't know how long it will stay at that price.
Now if you really care about multiplayer, meaning fighting against other players like in COD and Halo, then you probably aren't missing much unless you are really into those types of multiplayer.
I myself don't really like the campaign, but when I play this game with friends, do challenge with them, do quest, and do the Raids with a group, I have a lot of fun.
If you are not into campaign, or playing co-op with friends, then the game might not worthed, unless you really like COD or Halo type multiplayer fighting against each other

Buy taken king, original Destiny is a joke.

Taken king is what Destiny should have been day one, they just wanted to charge you twice for barely a game.