Some of the crazy low edgeguards you might have seen M2k recover from could be using side-b. If someone (usually a spacey) is directly below the edge, you can drop off, side-b them out of their Firefox/bird, double jump, and then up-b to grab ledge. I use drop back-double jump-bair-up b from ledge pretty often, but Marth has a super strong edgeguarding game from stage as well, so it won't always be necessary to go offstage, you have to judge each situation independently. You can get early kills be aggressively edgeguarding offstage, but if you start getting too predicable, you could find that your opponent will figure out how to turn what was an edgeguard for you into a kill for them.