Do you mean nothing the "buyer" can do about it?

,,,nothing anyone can do about it, other than eBay

How can I guard against this as its really annoying that they haven't really sent the item when they say they have.

The seller doesn't say they've sent it....it's the eBay algorithm assuming that they have

The seller as was said

That is how eBay set the system up

Seller (yes, seller) uploads tracking

eBay considers posted

Are these items  meeting the estimated delivery date (inserted by eBay, based on their guess)

Are the sellers meeting their handling time?

Sorry if I am confusing you, but Australia Post says it has given the item a tracking number.  but it just sits there.  I contacted them and they told me that people can get a tracking number before Australia Post physically has the package.  They even suggested it was probably coming from overseas but are not telling you that.

Yes, I buy satchels in bulk, so, I could for instance upload tracking at midnight....eBay then assumes that I've sent it

Sometimes it means they have put the parcel in a red box, so it won't be scanned until it arrives at a sorting centre, but it can also mean the seller has satchels with tracking numbers on them, and a tracking number has been assigned to your order, but the item has not been physically posted yet. This happens to me all the time, but after a couple of days the tracking does update to "received by Australia Post" when they actually go to the post office.