Scenario :
You have a quarter panel from one of Brocky's racecars. It has a signature on it from the man himself personally naming you. Someone offers you $1,000,000 for it and you wouldn't sell it?
Pffft, be real. A million dollars is a million dollars, regardless. The same as in that eBay scenario with the painting, $110,000 is $110,000.
At the end of the day it's his painting, his quarter panel, his whatever. He can sell it to whoever he wants for whatever he can get. That's part and parcel of living in a free country.
Whether it was signed by Peter Brock, Steve Irwin or the Pope himself, if it was mine and someone was willing to give me a bucketload of cash for it, they can have it.
[EDIT : Of course, all this is only my opinion]