To add to what GP has said,

1. eBay will opt sellers into the GSP without their knowledge, so they are likely to be unaware their item is going overseas.

2. If sellers input weight and dimensions of the item the postage usually drops to near normal postage.

3. There's nothing stopping you asking the seller to end the listing and send it via their own county's postal carrier - of course they have every right to not do that, especially if they do not normally post overseas.

4. You are wrong about the 100 pound limit on taxable imports, ALL imports have GST on them up to $1,000 when import duties become payable.