it's only going through regular USPS priority shipping anyway but because it's the global shipping program it costs almost double what it would have cost the seller to put in the mail himself.

Items are sent by airfreight, not USPS. The name International Proirity Shipping is misleadingly similar to Priority Mail International but there is really no connection.

As ebayers in USA pay their ebay fees on Item + Postage anyway it really won't make much difference to them whether they make the  postage  cheaper and you bid more on the item.

US ebayers pay fees on their lowest domestic shipping cost. I can't see how it follows that they don't care if they wind up with more money in their pocket.

It is certainly absurd what items some sellers offer with GSP, and how ignorant they are about it, but this is a consequence of the lamentable way sellers were opted in by stealth, and the near total lack of education about how to use the GSP effectively. It is pretty much like recruiting airline pilots by picking anyone over 5 foot high and letting them get on with it.

For Australian buyers, heavy items (over 2 kilos) become progressivly better shipping value the heavier they get. Anything under 2 kilos will always be a bad shipping buy, since a competent seller would be sending these by basic FCMI airmail.