I have always used Future's Market. I usually like to back on Wave 2 (to avoid cancer jungler) specifically assuming I get all the creeps then through that buy armor and a control ward. But when up against someone playing fun ranged magic damage like Teemo, you cannot buy MR on the second wave because it costs 450, not 300.
Why?
Because it's more impactful since AP champions suck balls compared to AD ones.
Why are you backing on wave 2? The second Teemo autos you a single time the wave pushes to you
Ya 
is +25 MR while 
is +15 Armor
25 MR is way more Def to AP then even 25 Armor is to AD let along 15 armor.
Cloth Armor (300 gold +15 Armor) is the item from which the armor gold value is derived, and as such, it derives a value of Gold 20 per point of armor.
Null-Magic Mantle (450 gold +25 magic resist) is the item from which the magic resistance gold value is derived, and as such, it derives a value of Gold 18 per point of magic resistance.
Mantle gives 10 more points and thats pretty much it. Aside from all the "magic damage hits way differently than attack damage" crap, they are actually pretty close(within 2 gold per point) in giving you what you pay for.
If youre asking why there isnt a 300 gold mantle that gives the same amount of mr as cloth armor, its due to the interaction between abilities and auto attacks which is a ton to explain lol.
You shoulda just went with I said, it woulda been more fun.
Magic users get shut down more by MR than physical damage gets shut down by armor.
Teemo is an exception (although in early levels I'd wager he's doing physical damage with auto attacks...), but most magic users rely more on bursting you, rather than sustained damage, to get a kill.
Why not alter things a bit against teemo? If you really must back at the second wave because of junglers (although against teemo he'll push you in, so I don't see why this is a threat), you could start with a refillable and rejuv beads, then buy MR with the leftover 300g on your first back. (Or come up with another plan that leaves extra gold so you can afford null magic mantle - also starting Ruby crystal could be an option).
It's to stop people from starting null magic mantle+multiple pots in mid. They'd bascally be immortal vs shallow early game mana pools.
Against teemo, take Second Wind in your runes and start 
. It will mitigate a massive amount of damage since both proc continuously through his poison and don't cost a 
charge.
Soon as you can get a 
you can basically just ignore his trading attempts since he's risking more damage from minion aggro than he'll be doing to you.
You don't have to be a tank to build 
, and it was made as an option for midlaners against DoT opponents as well as for tanks, so feel free to add it to any top or midlaner that can appreciate the matchup advantage.
