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.