D-tilt Trap
Might as well start with a bang. Marth's best trap and the second best trap in the game after MK's d-tilt trap. As you may or may not know Marth's d-tilt comes out in 7 frames and has IASA frames. IASA stands for Interruptible As Soon As. This means he can cancel the ending lag of the d-tilt when it reaches a specefic point in it's animation. For this attack that point is when Marth retracts his arm. The exact frame that you cancel at is 21. Normally the D-tilt animation last 47 frames. So when you cancel it the recovery time is reduced by half. This makes it an amazing shield pressure tool and one of the best pokes in the game. The d-tilt trap abuses these qualities for an almost airtight trap that an opponent is hard pressed to get out of. Doing it is very simple as well.
All you have to do to start the trap is space a d-tilt as a poke. The trap is initiated when your opponent shields or spotdodges. After that Marth has options to either punish any action his opponent takes or at the very least he will be unharmed and in a position to reset the trap and/or stay in control of the match. Now I will list each action an opponent may take and the appropriate counter.
Opponent rolls behind Marth - Wait for the roll and punish with Dancing Blade.
Opponent spot dodged the D-tilt - Punish with Dancing Blade when they come out of the dodge.
Opponent SH's back - Run towards your opponent and do a well spaced SH fair or just run into Dancing blade. You can also just walk towards them and reset the trap when they land or block and punish if they tried to attack when you walked towards them.
Opponent rolls away - Walk towards them and reset the trap.
It's clear that the two best options are roll away and SH back. Most seasoned veterans will roll away since it's the safest and fastest option. But if you expect that you don't even have to wait for the roll. You could just d-tilt then cancel into a full run and then grab or Dancing Blade while they are vulnerable from the roll. Your opponent could also try to shield drop and attack you or jump from the shield and attack, but these options are very risky and easily punishable by Marth. Good players will not attempt to do this. R.O.B and DK might try to hit you after a blocked d-tilt but if you space it at the tip, and watch for their reaction you can block their attack and retaliate. Projectiles users may also try to SH back or shield drop into their projectile, but again, it's a matter of being able to make split second reactions to your opponent's actions. Needless to say it will take alot of practice to fully master the d-tilt trap, but it is worth the effort.