the problem with that is..
They dont issue the brown slip
Its not their problem
Its like holding me responsible for a repair failure thats failed because of a defective part when I've done the repair using parts that you purchased and provided instead of ones I provide.
If registering vehicles was their trade then they would have a problem but as it is legally and morally it is the responsibility of the agency that is being paid to provide the service of the rego slipping paperwork to do their job correctly.
Similarly the reason you are not permitted to start, run or invasively inspect the gear (hell I'm surprised they let you open the bonnet, the one of the townsville auction houses wouldn't let you open the doors or get underneath) is that it is NOT the property of greys for them to give that permission, why do you think that there is a premium paid on every item they sell? Clever machine sellers in Darwin used to be on site when their gear was being sold so that interested buyers could at least run and test the basic functionality of the machine (this was 10+ years ago now I have no doubt that insurance has put paid to that) unsurprisingly the bidding was always much hotter on gear that the owner had shown up to permit testing.

