I'm not an Economics major, and frankly I've been doing too much math recently to think this through logically, so if there is a major gaping hole in my logic, have fun bashing this.
Let's first presume that you had unlimited access to keys (just for the moment, we'll take care of that at the bottom), and that you made it known on basically every medium available that you were selling keys for 1.5 ref. You also emphasize the fact that anything worth more than a key is not worth anything to you; as in, you will sell 3 keys for 4.5 ref, but a Genuine Point and Shoot will get them nothing. Now, over the course of a bit (who knows how long), basically every hat trader on the internet would eventually hear of this trader who is selling hats for 1.5 apiece (note: you actually do sell them to anyone who asks).
So what does this accomplish? Well, there are a few obvious effects. People trying to sell keys for more than 1.5 are going to get shut down pretty fast. With so many keys on the market, at some point their value will decrease even without your sales. With so many keys, people will start using them more and more on crates, throwing the price of stranges down the drain.
Also, until people really start figuring out what's going on, a lot of traders will rush to sell their high-tiered stuff for a TON of metal in order to get tons of keys in order to get even higher-tiered stuff. But.... if everyone knows that metal is now worth more because it can buy keys, no one will buy hats for keys. And if someone has enough metal to buy an unusual, they will have already exchanged it for keys, so few people will carry the materials to buy any decent hats and miscs, let alone unusuals. The result: everything decent in the economy will be practically unable to be sold, and in essence, worth jack shit.
For everyone that figures this all out, they will pounce on this opportunity in the first week or so, and sell all their hats and keys for metal, stock up on keys, buy the hats they've always wanted, maybe save some keys for unboxes, and then get out of the economy. When this whole thing is over, the economy will be irreparably destroyed, so people will just get the hats they want and then accept those as the last hats they will buy for a long, long time. tl;dr: the hour of reckoning for the traders and the economy itself will come.
But how the hell would you support this? Obviously you would need a few accounts, a lot of time, and some extra people to help keep this going. But the real constraint is money. How would you get these "infinite keys"? Well, let's run the numbers (I am good with numbers, but I haven't fact-checked these against the real statistics, so I'll be as generous as possible). Assume there are upwards of 400,000 people who play TF2, of whom 25% care about the economy. But let's face it; by the time this plan is up and running, at best 6,000 people who care enough about the economy--and have the resources--to fuel this dastardly plan will have bought in. Now, considering that we only have to worry about the metal they have, plus a bit of inevitable smelting, and maybe some good hard trades, each of them will have around 10 ref lying around. NOTE: I don't care about their Bills or their Blazing Kabutos or their Cloudy Moon Team Captains, because once they figure out that they need to sell those ASAP, it will be too late. So somewhere around 60,000 ref will be flying at us. 60,000ref/(1.5ref/key)=40,000 keys to get this plan together. 40,000keys*($2.50/key)=$100,000.
Alright, so that is a bit tough to manage. But for someone devoted enough to pull this whole thing off, and theoretically with the resources, it isn't an outlandishly high number. Never mind that Valve would become so rich they would probably give you anything you ask for, when you think about someone who would topple an online economy for fun, $100,000 would probably be chump change. A stretch, but to wreck an economy of around 100,000 people at $1 a person would be pretty damn funny.
So...... anyone want to start collecting?