Aloha and Welcome! :wavey:
To answer your question about buying pins, well, unfortunately the only way to guarantee a pin's authenticity is to purchase it directly from Disney (WDW, DLR, DisneyStore, etc). Does this mean that Disney is the only way to purchase authentic pins? No, of course not; there are plenty of honest resellers out there, but you just need to use common sense to distinguish the good from the bad. Like in trading, if something seems too good to be true, it probably is, don't be afraid to ask questions, and never agree to something unless you're 100% comfortable with it.
I don't know anything about the site you're asking about, but that doesn't mean anything; there are many sites (both good and bad) that I'm unfamiliar with. If you want to purchase pins, you also can try looking on the sales forums here, but please remember the same things I told you above to keep in mind (and don't forget to check references whenever you can).
I hope this helps and, again, welcome. 🙂
