Did you consult a tax attorney on this? I have no idea what you're referring to when you claim you don't pay "extra" taxes on selling your stamps. Selling your possessions is certainly income and would at least be subject to income tax (and the rate would be higher than a capital gains tax rate on stocks).
I'm from Germany, here they tax gains in stock trade with a higher rate and the government tried to extend this higher rate on trade with stamps and other "art" formats multiple times, but realized its unfeasible and for now gave up. Currently you pay a whopping 25% in taxes on capital gains over ~1000$ and have other substantial losses. A few years back it could even climb as high as 50% if you were unlucky. Also you don't pay any special taxes here if you simply sell your private collection.
I think the current US tax rate on capital gains is at...
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.