Postage from China (especially "free") tends to mean economy postage, and economy postage = cheapest way possible. Cheap = slow.
When you buy from the US, most sellers will use a USPS postage service, which means the package will come to Australia directly from a nearby distribution centre (near to the seller, I mean). If you buy from China, without upgrading to a more standard service, the package will often go through several distribution channels in bulk with tons of other packages, and can land in multiple countries before it gets to Aus. It's very common for packages from China to be routed through Singapore at least.
Just as an example, I use an economy shipping service through DHL when I post to international destinations, and those first travel to Sydney (I'm in Adelaide) for sorting and dispatch, and will either go to New Zealand or Germany before being sent on to the final destination country. This means they effectively go through 3 customs clearances (eg leaving Aus, arriving in / leaving NZ, arriving in US). If I want the package to go directly from me to the US, the package will cost about 3x as much to send.
I buy from China a lot, it doesn't matter what platform you buy on, what matters is the postage service the seller sends through. My personal preference (when not using express couriers) is for them to use UBI, it's economical (IMHO), and still has multiple distribution, Aus Post gets paid because they become eParcels when they arrive, and usually takes 2 weeks in normal circumstances (currently taking 4 or so due to covid etc).
