You will pay appropriate duty and ON HST on the laptop as well as whatever shipping is.
http://www.cbsa.gc.ca/import/postal-postale/duty-droits-eng.html
When Canada Post delivers the parcel, you will be charged the duty and sales tax, plus a $9.95 service fee.
If the seller uses a courier the same duty and sales taxes will be charged, but the service fee will be $25 or more.
I bought my MacBook a couple of years ago brand new for $1100 including tax at the Apple Store. $800 before other costs doesn't sound like much of a bargain.
Also.
If things go pear shaped, you would be required to return the laptop to Australia with Confirmation of Delivery and Signature.
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
Depending on the reason for the return, eBay may or may not ask the seller to refund the return shipping. However, they have no way to enforce this.
