I believe that, and I do wonder if it’s happening on a larger scale too. @KallDrexx suggested that people are creating fake games to give out keys and then farm for cards to sell for profit. I don’t understand who would be buying the cards for the fake games to allow them to make a profit, though… Unless it’s money laundering.
Put money into Steam, filter it through random cards to break up the trail, eventually buy keys you resell to pull the money out. Given the sheer volume of cards apparently trading on Steam, maybe that’s not too far-fetched.
Just in case anyone is interested, the average trading volume for the first 36 games for which I have cards in my inventory is 80 cards in the last 24 hours. They’re all pretty cheap, so that’s $4/card/day. Searching through the Steam marketplace shows roughly 30,000 cards with more than 100 active listings. If my 80 sales per card @ $0.05 average holds true (and it’s probably way too high), that’s $120,000/day. That’s only $43 million a year, though, so probably not actually the result of serious money laundering.