Why take Greater Mark of Attack Damage?
It can allow Poppy to last hit well.

Greater Seal of Armor is a must-have on most top lane champions, it allows them to take more hits to survive in lane, this is great against lane bullies, like Riven, or Nidalee.

Greater Glyph of Scaling Cooldown Reduction is a great choice on Poppy, it allows her to lower her cooldowns on her spells since they are fairly high.

And Last But Not Least... Drum Roll Please!

Greater Quintessence of Health Regeneration!
This allows Poppy to be fairly tanky in lane with more health regeneration, this is basically a free Rejuvenation Bead.

These are the core/most important items on Poppy.
Trinity Force gives her lots of good stats, for example: Ability Power, Attack Damage, Movement Speed, are all great on Poppy

Blade of the Ruined King lets Poppy do more damage on her Devastating Blow and auto attacks, add that up with Trinity Force, and you will be dealing TONS OF DAMAGE!

Black Cleaver lets Poppy shred armor on auto attacks, especially with Devastating Blow, lets say the enemy has 50 armor. You will be shredding 25%/12.5 of their armor on 5 auto attacks.

Boots

This will give Poppy armor, and reduce auto attack damage.

This is great on Poppy, especially against an AP top, it will allow you to take more damage in fights and in laning.

Ghost is great on Poppy, it allows her to dive in and out of fights, along with Trinity Force, Ninja Tabi, and your Mercurial Scimitar.

Ignite is also a great choice on Poppy, this will let her pick up easy kills and maybe a FIRST BLOOD, it also reduces healing effects of the target for the duration.