That's also assuming the people really want TF2 keys. The real draw of TF2 keys is that you can trade them through the trade window AND then turn around and sell them through the market. It's more convoluted than Paypal, but it's less risky.I really don't understand this. If someone wanted TF2 keys, why wouldn't they just buy them themselves at the rate you would buy them at instead of trading away something more valuable? I don't even know what TF2 keys do or which ones to get, but it seems like you are trading $6.45 for $12.49. How does it work?
The other difference is as refusedchaos pointed out, most of them aren't trading for something "less valuable." Thanks to regional pricing, they can sell for less than your Steam and still manage to turn a profit.