I've just sold a 2009 VE Commodore on Saturday, no problems. The car drove a 2000km trip a week ago. Now, 2 days later, the buyer is saying it needs a new gearbox for $2500 and threatened legal action.
I assume you gave them a chance to test drive and seek a mechanical inspection, and that you have no knowledge of any gearbox problems. Tell him you cannot/will not accept a return.
They did test drive it, than bought it and drove it home on Saturday/ drove on Sunday no problem. They rang me Monday saying the gearbox is stuffed and want me to pay half the repair bill.