I agree. It may be easy to avoid paying tax on the gain in value, but that does not mean one is complying with the tax law.
That said, if one is holding stamps as an investment, it is plausible that gains would be taxed at the capital gains rate instead of the ordinary rate (in the US).
Disclaimer to any reader: I am not your lawyer. I am not a tax lawyer. I didn't do any legal research. Don't rely on my opinion for any reason. Definitely don't rely on this opinion to try and pay less taxes. If this is a real issue for you, hire someone to do the research, or do it yourself.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.