The different in size names between Australia, the US and Europe causes endless confusion and probably causes many lost sales. EBay requires you to select a size from a list, and often sellers list the item as per its tag. Unfortunately, a European branded suit (Zegna, Boss) often uses European sizes and is thus listed in the wrong group. People looking for an Australian 40 will miss the same sized suit listed as a European size 50.
As a hint for buyers, look at both groups. For sellers in Australia, list according to the Australian group and put basic measurements like waist, inner seam, chest in the ad to avoid confusion.