There are US sellers who find just posting everything to Kentucky to be extremely convenient so they don't seem to mind the loss of overseas customers.  I can see their point, especially as they have no idea what the charges are to overseas buyers.

If I see something I like (and most of my purchases are of fairly low value) I will ask the seller if they will consider using USPS First Class International and I always mention the actual GSP cost to Oz in my request.  So far, some have said no, some don't know how to change it, some haven't even realised they have signed up for the GSP and they can be quite annoyed to find their listing is not 'Posts to US' only (because they don't sell to overseas buyers).....and some have been very obliging and changed the postage so I can buy the item.

For sellers who are worried about tracking parcels sent by USPS First Class Int, if their parcels are sent using USPS E-DELCON they can be tracked both at USPS and Aust Post (I'm currently following the progress of an item which I know has now left the US and is currenly en route to Oz).  Mind you, how well it is trackable probably depends on whether the seller lodged it at a post office and how efficient the PO staff are (I'm guessing....I'm not a seller let alone a US one) as the info can vary.  The last item I received was tracked every step of the way both here and in the US, though ironically it was the USPS site that had my item marked as delivered (which is was) before Aust Post site did, which was kind of weird, 😉

I think it's always worth politely asking a seller if they can post by USPS, but if they aren't willing to then fair enough.  It's their right to decide which method to use, and in that case I thank them for replying to my query....and I walk away from the listing and delete it from my Watch List.  It may be annoying that they aren't willing to change the postage, but then again, as I'm not willing to change to accepting the GSP then we are never going to see eye to eye, and my request may be equally annoying to the seller. 🙂

Still, there's no harm in at least asking.