Armor Pen Marks and Quints for obvious reasons.

Armor Per Level Seals = So you can resist 50% of the damage your gonna be taking in the 5-8 secs it takes you to kill someone

MR per level glyphs = same as above except for magic damage.

This build is about raping peoples faces before they can do anything, so you put alot more into offensive masteries then other builds

Must have:
Ghost: its your escape mechanism and can also be used to sneak around behind an enemy to do a pincer attack with an ally.

Optional Choices:
Exhaust: You spend the skill point for improved exhaust so you might as well get it.

Ignite: If your team lacks an ignite and the enemy have a mundo or chogath, you should grab ignite even though the exhaust skill point is wasted, it also really helps with flashing/fleeing enemies.

Start with a longsword because its a piece of madred's razor, but you dont want to rush the razor part of it, as your not jungling so there isnt a great to be able to do extra damage to minions.

get boots of speed next and depending on how your doing either get mercury treads or recurve bow, if you get recurve bow, then finish up mercury treads then buy the pickaxe, and cloth armor then finish the razors, then finish the bloodrazors.

next is malady, with each hit now your madreds will be doing more damage, you'll evaporate squishies at this point, and with some help probably tanks too, except rammus, rammus will rape you if you try to fight him... i think.

Sword of the divine and Youmoo's ghost blade are next for obvious reasons, their basically core items on xin zhao, and they'll work good with you already dealing magic damage at a crazy pace.

Now this build leaves an empty inventory slot, beccause i;ve never gotten enough time to build a 6th item, so here are some good choices:

Warmog's: i've always wanted to get around to building this at the end just because this build is incredibly squishy but they games never last long enough for me get it.

Frozen mallet: You shouldnt have too much problems with fleeing enemies, but this item would mainly be bought for the extra hp it gives yous, but its 250 gold more than warmogs is.

Stark's: Lifesteal to help you last a little longer in team fights. the only thing is your already at the ATK speed cap, so this is kinda a waste, but giving your friends attack speed and lifesteal is always appreciated.
An Executioner's calling might be good too, so you can have an ignite item if you dont get ignite, but that is kinda of alot of item actives.
The reason i dont think a bloodthirster would work is because even with all this attack speed and damage, your not a great farmer,so its hard to keep the stacks up on it since their all lost on death, and you will die quite a bit, but you'll take 2 or 3 of the enemy with you.