Ok Scar, I've watched your first match. I think you already know you have to use f smash much less. lol So I won't touch up on that. Three big things stood out to me that I feel you need to work on. Spacing, speed, and edge guarding.
Remember, the goal of Marth is to hit your foes with the tip. Out of curiosity are you using forward on the analog stick + A button for fairs? If you are then stop. You should simply jump, start holding back on the analog stick (to prevent Marth from moving forward) and then flick the C stick forward to do your fair. You'll land tippers far more consistent doing it this way. Another thing was your dash canceled down tilts. Just practice hitting the tip with those as well. A tipper from one of these at low percents will stun your opponent and allow you enough time to dash in with a JC grab.
Next was your movement. Your Marth was lacking in the speed department. Like Falcon, Marth can dash away immediately after SHFFL aerials. So get into the habit of dashing away after all your SHFFL aerials that either miss or hit a shield. For even more speed, get into the habit of wavedashing on the edge for the edge guard. lol Your dash dancing can also use some work. A cool trick I like to do with Marth is dash dance and then approach with my shield up. This sometimes makes your foe "flinch" which allows me to get in a free grab.
Finally was your edge guarding. There is an ideal spot you want to be standing when edge guarding with Marth. You want to be close enough to the edge so that when you do a down tilt the very tip of Marth's blade goes slightly off the edge level. This spot is perfect because if your foe managed to evade your down tilt, all Marth has to do is simply stand up and he's in perfect spacing for stationary SHFFL fairs and nairs. So if you your opponent simply stands up on the edge, they are going to get whacked by a nice forward smash, or tipper fair/nair. If they roll behind you, just turn around and grab them. So do keep that distance from the edge in mind...its extremely helpful when edge guarding. Oh and don't be afraid to run off the level and fair your opponent to steal their jump and what not. OR, you could run of the level, follow them and instead of attacking, just reverse Up B back. This may make you foe fall down a little too low and use their recovery a little later.
I hope I helped dude. It was an honor giving you tips.
Do keep us posted on your Marth progress. XD