I hate this matchup and don't know how to play it, but here's what I do know
1. Jab is good against Marth. You're supposed to jab him when he tries to grab you.
2. Usually, you want to grab the edge against Marth and make him upB when you're invincible. If he tries to grab the ledge, he's dead. If he goes back onstage, I would think a reverse nair (sending him back offstage) or waveland back onstage to grab or smash attack would be optimal.
3. If Marth is edgeguarding you onstage, sweetspot the ledge with upB or whatever. However, if Marth grabs the ledge, he is making you choose between sweetspot and stage-land. Marth's dair can cover both of these, so try either recovering high if you can or hugging the stage in order to tech the dair
And thats all I know. Hopefully Cactuar can give you a better response