Your method overlooks the fact that this will introduce an unhealthy number of new keys into the TF2 economy, thus causing them to lower in inherent value, thus creating a downward spiral wherein the disparity in values between BTCs and keys grows further and further apart. With that said, I've never played TF2, so I could be missing something.
Well, the value of the keys isn't a huge deal. They are only temporary tokens. It's only an issue if you are saving your wealth in them, haha.
The value of keys are a big deal. If there is an abundance of keys in the economy, people will start selling for less than 2.5 through third-parties.
If keys are now buyable for 2 dollars from a third-party, and people are still trading 2.5 USD worth of bitcoins for them, I know for sure I'd buy up all the 2 dollar keys I can. People would get screwed and caught with devaluating keys.