@chriss2804, the question of whether eBay's collecting GST on your international purchases is legal or not isn't a technical question, but is probably best posted in the Buying board.

Technical questions relate more to how the eBay site is working (or not working), asking how to get around a technical glitch (such as inability to log in, pages not loading, payment error codes coming up, etc.

Anyway, as my previous post has vanished (and thank you, lyndal, for being the second "witness" who saw it!), your specific questions are addressed in this post in the Buying board. Read this post for the link to the ATO's summary page for low value imported goods GST. Read this post and this post for excerpts from eBay's User Agreement referencing the GST for low value imported goods. See here for the post linking to and addressing the Law Companion Ruling LCR 2018/1 (GST on low value imported goods). And finally... direct link to Treasury Laws Amendment (GST Low Value Goods) Act 2017.