The main thing I'd say is that the ledge is not a good place to be against Marth generally (unless of course he's offstage) - if your opponent's spacing's good he can kill you with the tipper as you try and get off at some insanely low percents (this is why you should probably ban Yoshi's), so when recovering land onstage if you possibly can. You have to be kinda defensive in this matchup a lot of the time cause he can outrange you, but once you get in by punishing a roll or a missed fsmash or something you can really make it count - just get him in the air and carry him offstage with bairs/fairs. If he's above you, especially on a platform, you can get some great mileage out of upairs too.
Overall I think this matchup's a lot harder than people think (as is the case with a lot of Puff's matchups, unless that's just me being a scrub) so just be patient, wait for your opportunities and try and get a read on your opponent's movement.