[alanint]

The stamp itself is rare enough in its circulation that stamp collectors will often pay up to its face value to obtain one. The downside is that you can only technically sell a stamp once you have divested yourself of the firearm it taxed.
The underlying intent of the NFA act of 1934 was that every time a weapon changed hands, the current owner would roll the $200 tax into the sale price of the gun, eventually pricing them out of most people's reach. $200 was a whole buttload of money back in 1934. While the $200 has become a more or less acceptable cost for a transfer in this day and age, other market forces have exponentially pushed the cost of such guns up, thus for all practical purposes fullfilling the original underlying intent of the 1934 NFA act.