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Pro Massive catalog
The stamp catalog is massive. It lists not only countries, but also provinces within countries to easily find the stamps you're looking for. Altogether, there are 750,000+ stamps listed.
Pro Social experience
StampWorld lists events, and also hosts forums, clubs, and organizations through its website. If you want to, you can meet up with like-minded people easily.
Pro Built-in marketplace
You can browse for new stamps, or sell old stamps easily through the website. There are also dedicated spaces for auctions and dealers.