It's hard for me to talk about how to fight Marth because I have my own system and flow chart of things to look for to know they are vulnerable. It's not really something I'm conscious of, just something that happens from years of fighting really gay Marth's who know the matchup.
The big thing I would focus on is learning the windows that Marth is vulnerable, and learn to exploit the hell out of them. He's combo fodder if you get underneath him (and without his second jump? forget about it), and even tho pills are typically swatted away, spamming the hell out of them is very key, just don't compromise your space and try to keep the middle ground. Platforms def help. Dsmash and bair are your best friend.
As for the best way to damage Marth out of dthrow, at very low%'s I just go for dthrow -> usmash. It's a free 25%+ damage, launches marth above you for more potential follow ups, and works until like 40%+. One thing I've started doing recently at mid % is dthrow -> dair -> something, which works incredibly well and has the potential to link into a smash attack out of an uair from the dair, or straight from the dair into a dsmash or something. Really really good, especially with platforms.
I have a few more secrets, but I gotta save those.