My wife is a lawyer and she buys a lot of stamps. Recently she stumbled onto some strange stamp firesale on Ebay and bought forever stamps at below face value. I could only guess that this was an unwinding of the trade you propose.
It goes like Ebay says spend $X or sell $X on ebay and they'll give you a rebate. (Or sometimes the rebate comes from a credit card or Bing). If the $X covers ebay fees and shipping, then buy stamps to reach the $X and then sell them when you get them. If the rebate it sufficient, you can sell the stamps for less than cost, because you're already ahead.
For a cash-like instrument, stamps seem to have very little in the way of anti-counterfeiting security.