Graves is a devastating ranged carry that deals more damage when up in the opponents face. Since it is beneficial to unload your shotgun into the chest of the opponent, I like to build him semi tanky... Here is a build I have had success with, let me know how you like it.

Greater Mark of Desolation Armor penetration for increased damage
Mana regen per level, as Graves is mana hungry and I do not build any mana regen items.
Magic grant a bit of magic resists, which is lacking in the item build.
Greater Quintessence of Desolation Armor penetration for increased damage

Optional runes
If you take thes rune, go 29/9/0 and take advantage of nimbleness. Also, Ninja Tabi becomes more effective at that point.
If you do not take take mana regen per level seals, I recommend taking the Glyphs
I think movement speed quintessents are useful and underrated, but I still believe armor penetration is more useful for Graves.

Buckshot is Graves bread and butter damage ability, so it should be no surprise that it gets priority... I like to increase quickdraw before smokescreen to decimate turrets easier early game, but the increase in slow potency when increasing smokescreen is not to be overlooked... In other words, if you are up against melee champions, it may be a better decision to increase smokescreen earlier.

Fairly straightforward, 21/0/9 taking improved exhaust and utility mastery to increase buff duration.