"It's just an old Toyota ute that someone has overpriced. Who cares if he drove it."
Mr Watson said he had not been close with his father.
Yesterday, at the time of going to press, the Trade Me listing had been viewed over 54,000 times.
The advert said the vehicle was "in original untouched condition showing its age and patina with significant rust around".
Film director Taika Waititi accused the ute's Trade Me seller of "cashing in" on his latest film.
Mr Crump wrote Wild Pork and Watercress -- the novel Waititi's latest film Hunt for the Wilderpeople is based on.
Waititi took to Twitter to express his anger.
"Please don't pay these blaggards 25k for a car full of rust. Nice timing, btw, cashing in on our film's release."
Mr Watson said it had been "unfortunate" his father had also been "surrounded by people on his coat-tails".
Mr Crump married five times. He died in 1996, aged 61.
He is remembered by some Wairarapa people as enjoying a beer at the Kuripuni Tavern while living in Masterton.
Mr Watson said he was not close to his five brothers, all of whom were fathered by Mr Crump.