DkDKDkDkDk, I watched your Marth ditto vid. Pretty good stuff, but there is a lot you can do to improve.
General Stuff:
Throwing, Observing, and Manipulating:
OK, right off the bat I noticed something that you can work on that will make you a much better player. You should never force something from Marth when you want it, it's generally a bad idea. If you don't beleive me, watch my match that is up in the Dr Peepee thread, when I try to force something to happen I get punished.
Immediately after you uthrow you need to look for one thing whether or not they second jump or not.
If they don't jump and they are at low percents, then just uptilt them. If they are at higher percents and too far away to uptilt I generally advise people to shorthop towards your opponent but just wait.
Usually this prompts your opponent to jump or perform some kind of stall (in Marth's case his forward B). If it doesn't and you are in range, uair/fair/bair depending on your position.
This is a great thing, because if they jump or stall then you have enough time to land and consider your next option, but your opponent is still stuck in the air with one less way of avoiding your attacks.
This isn't easy, but it's definitely the best way to go about it. Limiting your opponents options makes it a lot easier to figure out what they are able to do in a circumstance and gives you a higher chance of being successful.
On this same note, you kept forward and downthrowing him and always tossing out an fsmash. Again, you can't force things to happen with Marth, react instead. I know just how tempting it is to try and force a quick KO to one of your opponents stocks, but in most of these cases you would have been better off uthrowing and playing off of that. For a point of reference, out of all the fthrow/backthrow fsmashes you did, only one landed. So that is at least 4-5 wasted grab oppertunities that could have been a nice juggle that could have led to a KO if not a lot of percent.
I would say that this is you and your opponents's biggest error in play overall. You are both so eager to fsmash that you give up so many more better options that you have at your disposal. Fsmash is a great move, but it should not be spammed, and you shouldn't toss it out very often in a match unless you are certain it will land.
Spacing:
Both you and your opponent have pretty poor spacing in this match. If you or your opponent was spacing attacks well, it would have been a very different match. Against other characters Marth has a bit of leeway on spacing, but not against himself. Your spacing has to be spot on in order to punish your opponent from bad positioning or juggling. You can't afford to drop potential combos AND take a hit yourself when one tipped attack could spell out a stock. You should really spend some time focused on mastering the tipper range of Marth's aerials.
Try placing bomb-ombs on platforms in training mode. Put cpu's at each end of the stage so that you can see everything and then just practice spacing your fairs so that you can attack the bombs without them exploding on you.
Out of shield options:
There are several times when you are locked in shield and choose to roll instead of wavedashing, or took a hit to the shield and then failed to capitalize on the possible punish by not wavedash and grabbing your opponent. Marth's out of shield options are immensely important to learn, as rolling is fairly easy to punish. Practice to wavedash out of shield against computers after they land an attack so that it becomes something that feels natural to do. Learn when you can actually follow up on it with wavedash > grab/attacks.
Neutral Game:
Both you and your opponent toss out aerials and fsmashes from neutral all the time. You are much better off not approaching with aerials at all, and it's definitely unwise to throw out random fsmashes. That works right now against opponents your level, but it will only hurt you later on when people play a lot safer.
If you feel that your opponent will be in a zone for a random fsmash, try dtilting instead. Focus more on your ground game and using movement to create openings. Use dtilt more, it's an amazing move that you and your opponent underuse. Be sure to practice moving during your IASA frames and you can get away with a lot of nice pokes from neutral with it.
Risk reward:
Both you and your opponent are engaging in very risky maneuvers that are completely not worth it that cost you stocks. Try to focus on playing as safe as possible and don't try to hard read your opponent. Sure, you won't get flashy crazy kills this way, but you won't whiff so many fsmashes or die from being overzealous off the ledge either.
Stick to the ground:
You have a similar habit that I do in which you sometimes short hop for no apparent reason. You also full hop over your opponent whenever you are pressed against the ledge, even though you aren't really forced to do so yet. It would be very easy to punish this behavior if your opponent caught onto it. Focus on spending more time on the ground and spacing dtilts more.
Using platforms:
You could have used the platforms better in your match. Learn to waveland! It's a must, really. Also, focus on locking your opponent to the platform for as long as possible. Also be aware of their options while on the platform.
Keep in mind their position. If they are further away from the ledge then their options are to either drop down, or to go to the top middle platform. If they are closer to the ledge they can drop down, drop down and grab the ledge, or make a break for the middle platform. Always look for that moment when they begin to make their move and be there waiting for them.
Edgeguarding:
When edgeguarding from the stage there is very little benefit to get to the ledge as soon as possible. When you do this you forfeit a lot of potential options and leave yourself open to being vulnerable on the ledge after your invincibility frames wear off. It's better to stay on the stage close to the ledge and wait it out. If they are getting close to the stage and aren't coming from above then your opponent only has two options: sweetspot the ledge, or try to upB to the stage.
It's not a terrible idea, at this point, to take the ledge because this will force them to upB onto the stage. This often leads to a KO, or at least putting your opponent off stage again. If your timing is good though, you can wait by the ledge at the proper distance and simply wavedash > ledge grab whenver they have past the point of not being able to land on the stage.
Small specific things:
Whenever you wavedash off the stage to grab the ledge you need to press down right after you start falling. This allows you to catch the ledge near instantly, and makes edgehogging much more efficient and easier to time.
While falling, try not to dair into your opponents so much. It's very telegraphed and you get punished for it most of the time you try it.
Kudos and Props:
Your movement at 8 minutes is really good. Where was that in all of the other matches? You had complete control over your opponent when you used your movement to put pressure on your opponent. That entire match was so one sided because of the fact that you played safer and used your movement more. You relied less on random fsmashes (although there were still several of them) and you played more patiently.
Overall:
Watching you play is like watching me play around six or so months ago before I started working on being safe and playing more reactively. I think you have a lot of potential and that with a slight change of mindset you can improve tremendously in a short amount of time. Go and read up as much as you can in Peepee's thread and take in all that advice towards playing in this new way and you will be a much better player for it.