Edited out the part about using jab as a frame trap. It's frame data is horrible and doesn't lend itself to being used in such a manner.
Check it out.
Jab
Hit: 4
End: 35
IASA: 27
Shield Stun: 6
~Shield Hit Lag: 3
ADVANTAGE: -20
Tipper Shield Stun: 8
~Shield Hit Lag: 8
ADVANTAGE: -23
Yeah. TERRIBLE!!!!!
But I also looked at fair and it is pure SEX!!
Fair
Hit: 4-8
End: 34
Shield Stun: 10
~Shield Hit Lag: 5
ADVANTAGE: -25
ADVANTAGE (SHFF): -4
Tipper Shield Stun: 13
~Shield Hit Lag: 11
ADVANTAGE: -28
ADVANTAGE (SHFF): -8
SHFF Fair Lag: 8
SH Fair Fast Falled Lag: 3
Tipper SHFF fair is -8 which is amazing by itself, but non-tipper is -4. That's orgasmic. So if you space your sword so it's close to tipper range, but not quite you can still make good use of Marth's range and get the benefit of this amazing shield pressure tool.
BTW d-tilt is -7 on a shield but only if you don't hit with the tip.
Down Tilt
Hit: 7-13
End: 47
IASA: 21
Shield Stun: 11
~Shield Hit Lag: 4
ADVANTAGE: -7
Tipper Shield Stun: 11
~Shield Hit Lag: 11
ADVANTAGE: -14
So again, space it so your opponent is still outside your zone, but don't hit with the tip if you want to use the frame trap. If you just want to poke at them, then use the tip.
Ok, now I'm going to talk about Dedede. This one is pretty easy since he is so straight forward.
Dedede has no attack traps that I know of. He lacks the speed for that and the recovery time on his moves. What he does have, are brickwalls.
Counter measures
Waddle Dees, Waddle Doos and Gordos - Dedede likes to throw stuff. Due to his great weight and lack of speed, it's the best way for him to control the play field. There are ways around this though. You can swat them away and you will usually want to use fair for this or jab. This is an easy and reliable way of doing things. This way you can get rid of his control while still closing in on him and not lose any ground. You can also shield them, jump over them, roll or spotdodge. You have a plethora of options really. Remember though is that Dedede isn't throwing things to hit you. He is doing it to force a reaction. Always remember your spacing in relation to his as well as how vulnerable your actions will may or may not you. Keep in mind the execution speed of what your doing as well as the recovery time and whether or not it will put you in a bad position. Dedede will prey on you doing something risky to avoid his projectiles so he can put you into a bad position and take control of the match. Generally shielding and swatting are your best bets. It may take awhile to close in, but it's very safe. You can't swat Gordos so be mindful of that and keep your eyes open. At certain ranges you can duck under any projectile he throws and then close in during his recovery time. Keep this in mind.
F-tilt - This is another obstacle you must overcome. It's not very threatening honestly. Mostly just very very annoying. At very close range if this hit's you then Dedede is at frame disadvantage and you get free damage. This outranges all of your moves, but it's great range is mitigated by it's recovery time and low damage. You can SH over this and attack Dedede during his recovery frames. You can also shield it and punish with a shield dropped DB depending on how close you are. Those are your two most reliable options. You need to gauge accurately when you can punish this if you shielded it though. If you were too far you could just end up dashing towards Dedede and get punished for it. Be mindful of your spacing at all times.
Bairs - Dedede will mix in Bairs to keep you at bay. If he starts doing this, then you can wait for it and attack his foot or throw out Nairs here and there which will beat Bair head on if your spacing is good. That will deter him somewhat from spacing Bairs so much.
Alone these three brickwalls are decent, but good Dedede's combine them and mix it up to create one very effective wall. And when one mistake can mean Marth eats a CG for around 30% or more, you need to be on your toes and mindful of your actions and their consequences at all times.
Ledge traps - This is very easy for Dedede to do once he gets in the situation. All he really has to do is space himself so that he can grab Marth or f-tilt him, if you decide to do a normal get-up or get-up attack. A very bad situation to be in. Do some ledge drop re-grabs to throw off this timing. If he is close to the ledge then roll, and if he is farther away then do a normal get up. Ledge jumps are good, but if he calls it then you could end up eating a Bair so be weary. This situation is a gamble and you must watch Dedede. His habits and spacing will decide your actions.
What works and What doesn't
Attack traps - The d-tilt trap works fine vs Dedede and is great for limiting his options. If you touch his shield he will more then likely attempt to grab out of reaction or roll. The grab will miss and then you get a free DB and if he rolls then you are in no danger. If you know he well roll then you can chase him down and get a DB. don't attempt that unless you know he rolls out of habit because it could get you grabbed. The fair trap also works well, but you should stick with the tipper version of it. The non-tipper version might get you grabbed since there is less shield push back.
Juggle traps - Dedede is big and fat, but he has multiple jumps and a good Bair. You CAN keep Dedede in the air, but it's tricky. Don't worry too much about being super aggressive if you get him in the air. Just make him feel pressured and waste jumps. Try to feel him out. You don't actually have to do much here. Do some empty SH's and maybe some SHFF Uairs. Just enough action to make him feel like he is danger. If he does something risky like a FF bair then you can punish. Remember not to put yourself in danger due to over zealousness. Doing that defeats the purpose of the trap. To put your opponent in a bad position where you are at little to no risk even if you don't get damage in.
Ledge traps - You can do it to Dedede just as well as he can do it to you, if not better. Space yourself so you are beyond his get up attack. If he does something easily punishable, then lay on the hurt. Once he wises up he will probably start ledge jumping or ledge drop mindgames. Watch him closely. There will be times when he gets way, but don't make it easy for him. You can rack up alot of damage when you get into this situation, so make the most it.
Brickwalls - Meh. Once you get past Dedede's walls, you pretty much have to set-up shop in his face and wall him yourself. That's pretty much the whole match. Watch your spacing so you don't get grabbed and CGed.
Throw traps - Dedede doesn't like f-throw. He is very heavy so once he gets thrown, it's an immediate 50/50 guess. Should he airdodge or not? If you call it, then you get damage. Don't do anything risky though. It's not necessary. Usually you will want to do a SH fair in this situation. If he didn't airdodge, you get damage, and if he did then you are safe from retaliation as long as you spaced well. Yeah, it's a really brain dead follow-up where you really don't have to think, but so what? That's fighting games for ya. Once you pick up on his habits you can predict him and get in damage easier. You also have a CG to DB or f-smash on Dedede at very low percents.
Ok, that's the Dedede write up. Is there anything that anyone want's me to add or talk about?