Runes are free, except the quintessences and seals, thet must be Greater Quintessence of Gold and Greater Seal of Gold. Why Avarice? Because they give us more gold and who we want is the gold. We love the gold and this runes are our friends, because they give us more gold!
We only need 4 items:
Heart of Gold: Give us survivality and gold. The 250 health is ok, but the important thing of the item is the 5 gold per 10 seconds
Avarice Blade: Give us critical strikes, thet we won't need, because we don't have damage, but we buy it because is cheap and give us gold
Kage's Lucky Pick: It gives us AP for Spirit Fire, but we are not going to buy this item for the incredible 25 AP that it gives. No, we are going to buy it for its more incredible 5 gold per 10 seconds
Philosopher's Stone: A rock that give us health regeneration and mana regeneration. For surviving is ok, but that make it amazing is that give... 5 GOLD PER 10 SECONDS! \o/
The rest of items are useless in Nasus, specially that Trinity Force, I don't know why is core items in a lot of Nasus build, it doesn't give gold, that is what Nasus needs.
If you have a lot of money, you can get this items:
Boots of Mobility: To avoid Team fights, you will need speed. And this boots give you speed, but you have to avoid to fight if you want the bonus speed of them.
Zeal: If you consider that Boots of Movility doesn't give you enough speed, you can buy zeals to have 7% of speed for running more. 4 zeals will be ok
Sight Ward: You will need wards, but don't put them in dragon, baron or other common places. You have to put the ward in the enemy base, where the enemy champions spawns. Why? Because you need to see if they are in their base or not. And if your mates have Teleport, they can gank the enemies in their own base (if the lazor tower don't kill them before)
Phantom Dancer: If you still have money and 4 Zeal, is time to upgrade them to Phantom Dancer to have more speed to run of Team Fights.
Lane phases: Be the jungler, start with wolfes, later wraiths or golems. Don't touch blue or lizard, they do a lot of damage. Maybe your teammates want that you go to gank, but you don't have to take risks: Your life is more important than theirs
Mid game: You can farm now in the lanes if there isn't no enemy champions, maybe they are all in Team Fight fighting versus your team. They will say you that you have to go to the team fight, but you are more important than them, you are rich and their are poor.
Late game: Your team is losing the game and the enemies are in the inhibitors. If your team want to surrender, vote no always. It's time to sell the gold items and change your build. You have to buy Boots of Mobility, 4 Zeal and a Sight Ward. If you still have money, get Phantom Dancer to get more speed. It's time to put wards in enemy base. If the enemies wants to kill you, you have enough speed to avoid them. Remember: The most important thing in this game is that you have to been alive
With this guide, you will be reported if you do this in normal games and rankeds, except you are with friends. I only reccomend this build when you are joking with friends in custom games or normal games, but if you go alone with this build, prepare to mass reports