Now this is a very interesting match up, i mean don't get me wrong Marth owns Young Link but....theres a very simple way to beat him and ill list them for you bro.
#1: DO NOT SPAM 100% OF THE TIME!! Smart Marths will see it and charge up to you, and when they do your screwed. Be extremely evasive with Wavedashing, and don't jump to him.....EVER!
#2: Camp Marth until he does his Fsmash, This will be your opening when fighting him so you can do your ultra grab combos to him. Also when your at a distances, then always keep a bomb in your hand, If he spams Fairs then as soon as he lands hes a tiny bit open so time your throws of the bomb.
#3: If you seen Laijin videos, then edge guarding him should not be issue. But they usually like to sweetspot, so to be on the safe side, just grab the edge.
These three things should make Marth not much of a problem, thats how i fight him. NEVER GO IN TO FIGHT MARTH! thats asking for trouble, wait till he comes to you, and if he doesn't.....spam =). Hope i helped.