UN authority is very limited, and largely dependent upon whether member nations agree to accept the articles. Even if they do accept any articles, it is not uncommon for member nations to ignore or purposefully snub their nose at enforcement or complying.
I think it's actually mentioned specifically in article one. A whole bunch of nations made declarations of provisions which either outright rejected certain articles, or declared they would not enforce the articles.
Since when did Google decide they were going to be the Planetary Police Force?
There are some nations that actively enforce the prohibition import/export of stamps more than 100 years old. Don't hold me to this, but I believe Italy is one of those nations.
Anyway, all the info is available at the UNESCO website if anyone wants to do a study of utopian lunacy. I'll dig up the link and post some of the pertinent parts/provisos.
Sellers/buyers should be aware a few (very few) nations actually enforce the rule. But they are not going to haul you off to face the firing squad. Your stamps will be confiscated, however.