I looked into that as I do buy quite a few items from USA sellers.  On the face of it, they charge the buyer to manage international posting costs + customs and import charge even though there are none of these for the second hand/used low price items I buy.

I have been having my USA won items sent to a friend in the USA and he just boxes them up and forwards them to me at postage cost only when he has enough to fill a box.

The GSP items do offer good seller protections for USA sellers that use the service, but the buyer ends up paying more to provide that protection for the seller as the seller charges the buyer to get the item posted to the shipping provider (Pitney Bowes) as well as the postage to get the item sent from the shipping provider to here + a customs/import fee.