this item is simply amazing on Teemo the health it gives helps you survive, the regenerative abilities allow you to stay out for extended periods of time and the mana for all of your skills as without mana regen Teemo can actually have some mana issues.
this multiplies your damage output allowing you to kill more, not much to explain on this one.
this item allows for the combo of Move Quick to proc Lich Bane then use Blinding Dart then another auto attack for the second Lich Bane proc; depending on the other items in your build this should nuke a good amount of champions from full to 0 very quickly.
just with this and maybe one other item it will take out about 50% of an enemy champions hp when they hit a Noxious Trap this item also gives Teemo other well loved stats such as hp, you can now use Blinding Dart to apply atype of poison to your opponent
these are the boots you'll want the majority of the time.
alright a good amount of you are probably wondering, why grail? well this item has some ap some cdr and even some mana regen in it for you, yea you already have roa for mana but i can't seem to find a decent cdr ap item that fits my playstyle therefore i take grail. this also gives some decent mr

These items are situational and should be treated as such getting only if one of the other items will not provide you with what you need.

the ap snowballing item, this is a very scary item to see on a Teemo as his Noxious Trap allows him to rack up assists/kills very quickly giving him powers through the stacks of this.
although i haven't really tested this item very much due to it's price, all of the stats it gives aid Teemo in some way or another. just remember if you get this do not get Frozen Mallet or Lich Bane.
i don't know what it is about this item but i just don't like it on Teemo it's a good item however it seems there are better alternatives for him, however if you want this item i recommend selling it late game for something better.(unless utilizing [[runaan's hurricane]
i used to get this item as all the stats compliment Teemo rather well however with it losing the mana regeneration part of it, it seems rather lackluster.
this will reduce (hopefully) how often the enemy focuses you. i used to get this as part of the build that utilized Mejai's Soulstealer to help protect the stacks it offers. (if you know this will be activated try to drop a mushroom before you die so that when you are reviving the enemies live will be draining.0
this item has potential as it gives massive ap,cdr and a powerful active to nuke people with, experiment with it if you like as this helps to take down tankier champions.
i usually only build this when the enemy has a Karthus as his Requiem can hit quite hard usually. this item has other uses however as it also gives massive ap as well as some armor, you could use the item kinda like the ga as mentioned earlier where you poison the opponent then hourglass for safety.
this item.... it's a very popular item however that doesn't always make it good, this item only offers a high amount of atk speed and a multi-target basic attack that applies on hit effects, this applies your Toxic Shot to 3 different people, i don't think it's worth the price unless you also use Malady and Wit's End
this is mainly for more dot; take these if you feel you aren't attacking fast enough.
this item is a great item for Teemo and it gives Runaan's Hurricane a better chance of being used.this increases your dot by a good amount as well as making you a tad bit tankier due to the magic resist.
this item is rarely ever bought on Teemo and i don't see why, it give decent ap decent ad both work well for Teemo it has a ranged slow so they have a hard time escaping you especially with your Move Quick skill, the spellvamp on it allows your Blinding Dart and your Noxious Trap to heal you and the lifesteal is nice for your auto attacks.
Teemo is well known for his Noxious Trap and these boots are all the cooldown from items in your main build that you'll receive, get these if you need more mushrooms early on otherwise they can wait a little bit as your tier one boots will suffice for speed.
more than likely your opponent will build some magic resist to counter your damage, if not then getting this will reduce their non existent magic resist further.
this gives a large amount of health and a built in slow for your auto attacks and even a tad bit of ad, this item is more for the survivablity and the slow late game as other champions will more than likely do more than your Rod of Ages could handle earlier.

I feel i should also mention that when swapping items to make sure to obtain near max cdr as Teemo needs to lay as many Noxious Traps as possible.