I understand that GST based on market value would have been returned to the ATO in the final BAS after GST was cancelled.
The question itself may be a little misleading and could be better phrased -
1. the company itself is not deregistered, only it's GST registration has been cancelled.
2. "sold the car that the company had bought while still registered for GST" might be misread as "sold the car (that the company had bought) while still registered for GST"
Otherwise, I would have thought the company would not be deregistered while it still held assets ?
CAPITAL ASSETS HELD WHEN A GST REGISTRATION IS CANCELLED
When a business enterprise cancels its GST registration, it may still hold capital assets on which it has, or is entitled to claim GST credits.
In this situation the amount of GST to be paid is subject to an increasing adjustment to reflect the market value and percentage of business use of the assets at the time the GST registration is cancelled.
The GST should be paid in the final activity statement lodged by the enterprise.
The GST amount payable should be 1/11th of the (percentage of business use times the market value) of the asset.