Trade gift games for items on steam (beta)! I know, you can’t trade used games. Interestingly, this will create additional mild demand for games, adding value as a currency (and increased liquidity).
I wonder if that means some indie games will become common currencies. Also, if I were feeling speculative, I’d open a fresh steam account, trade currency games into it, wait for their value to become inflated, and then sell the currency account on ebay to get a return on my investment.