I don't really see how it makes any significant difference and honestly I did it for free postage and 1 cent no reserve over the school holidays for a bit of fun more than anything. Plus then you don't need to worry about adding anything, what you bid is all you pay.
The Insurance is going to be the higher cost for myself than the postage, which is based on total value so sending 3x parcel worth $400 each or 1x parcel worth $1,200 is going to be the same Insurance cost for myself. Technically it will actually work out cheaper sending each coin separately as Australia Post covers the first $100 anyway.
The pre-paid letters I bought in bulk over 10 years ago so only cost me 65 cents and the Registered Labels I get for $2.50 each in bulk. Sooo I am more than happy to post all 3x coins separately to the same person if they prefer, that suits me better with the Insurance. I am not sure how it makes it 'unfair' to any specific buyer. If anything it would be unfair to someone who lives local and wants to pay cash on pick-up, that to me would seem more unfair.