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.
Stamps (forever or not) are often used for Ebay manufactured spend because they're easy to sell (clear value, lightweight and easy to ship).
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.
