From what I gather, these people are flooding the trade markets with keys likely obtained through illegitimate means (all the more reason to monitor your account data closely), exchanging them for buds, then selling them back for money (which the article points out these guys are getting 700 rubs each, which is the Russian equivalent to $22.71 USD)A group of Russians bought >5,000 Keys from Store today using illegitimate credit cards and bought few hundred Buds from TF2 Outpost at an ridiculous price of 28-30 Keys (The average yesterday was 25.5 Keys). They have been doing this for quite a long time and their activity today is alarming.
Here's the full post, conducted by a guy who probably has too much free time:
http://forums.steamrep.com/threads/toda ... ket.14096/
Just goes to show, if you openly engage in trading markets, keep an eye out for less-than-scrupulous buyers/sellers out there.