It was a different time in the heady days of the trading card beta. A foil would easily sell for 10+ USD, at the least. Fortunes were made and clandestine card control operations were formed, organizing and exploiting the differences in prices of games between regions.
A Russian contact was your best friend. On average, a game would sell for 66% (in addition to any other discounts) in Russia for what an America would plan to spend. Wait for a sale. Buy through contact. Use Steam Achievement Manager to idle game without even downloading to acquire cards. Sell. Profit.
But the market burst. Supply quickly outstripped demand. Like tears in the rain.
TL;DR You sell them for money