I prefer Trundle Off-Tank, so i start buying a frozen mallet for maximized hp.
I usually buy warmogs insted, but it's a little expensive and the damage dealt will be mutch lower.
I hope you enjoy, and please dont under estimate trundle, it's a bad idea...

I start with a doran's shield for more strength and hp regen, for more combact durability.
A frozen mallet, starting with giant belt of course.. it gives more hp, damage and a very usefull passive.
Then the phantom dancer. This is were the AD starts to grow. More movement speed, critical strike chance and massive atack speed.
Black Cleaver it's important, but if you are having trouble farming or at least surviving in team fights xD, I sugest you to buy stark's fevor first, because its less expensive and it's nicelly effective.
At last, Last Whisper for massive damage. (armor penatration).

Marks - armor penatration of course.. for more AD.
Seals - I prefer mana regen because, in my opinion, Trundle it's a big mana eater...
Glyphs - Magic resist to have more defense. (or you can buy greater Glyph of force, for coldown reduction)
Quitensseces - armor penatration for even more damage, more effective.

Take exhaust for chasing a champ.
I prefer Ignite for a low hp champion finish.

Rabid bite gives you more damage if you use it twice your damage it's even more, but this effect does not aply for more rabid bites...
Contaminate has a crowd control effect, so be sure to use it before any other skill, to prevent incoming stuns, snares, etc...
Pilar of Filth is great for a pursuit, if you cast it rightly positioned, it will bring the champ back.. and also slowing it.
What Agony has best it's the stealed magic resist and armor, so use it on a off-tank and set up your team to focus all atacks on him!

Well, I hope you enjoy this tank killer build xD
Remember, if you can't handle true power, you can't handle Trundle.
Don't forget to thumb up! ^^

Good games!