This really isn't the place to discuss this but there are a couple of obvious risks. If I send something internationally and it gets lost by the recipient's local postal service, there's nothing I can do if the package was sent by USPS priority or something that only tracks the package until it leaves the US. If the item gets damaged in transit, you can't even send it back to the person to get a refund without significant cost to yourself and the seller (see: The LZ-FE packaging fiasco). Heck, if I send something via first class mail and the buyer says it didn't show up, I can't even prove I sent something because there's no tracking at all for those items.
Like I said, I buy and sell internationally all the time and largely have few problems. But I can see why people wouldn't want to, and for that you shouldn't really criticize people.