I'm with Andrew on this one (man, I hate when he's right and I didn't get there first

). If your dad has a 'vette, he should look into selling it via a custom car collector and if the boat is that good, sell that too. I think an auction as you are proposing is a risk, but selling the car and boat is not risky at all (as long as you can find a buyer of course).
That should get enough money to stop the foreclosure I would think, even if the proceeds didn't cover the entire amount. I know it would be hard for him to give up those treasured items, but if it's a choice between the roof over my head and my classic car, it's goodbye to the car. Has he considered doing this? He could get a deal done pretty quickly ... likely quicker than an auction would need to run to get enough "bids" to help out.
To me, the online auction idea is too risky in terms of whether or not it would be acceptable to the website, plus you never know if the ultimate bidder(s) are legit themselves, so it might wind up being a huge waste of time and effort and get you no further to your goal.
You'd probably have more luck going to the local media with the story and see if some nice people come out of the woodwork (no pun intended) to help out. Just my two cents worth.