Some states require dealers licenses to participate in auctions. But if you know a guy with a license, you can score amazing deals (which are only amazing because of the limited pool of bidders).

I think knowing how to fix cars is valuable, but I don't think doing maintenance yourself generally pays off. I'm pretty handy with a car, but the only maintenance I do myself are brakes and batteries. Tires require special tools, so those are out for the shade-tree, same with suspension (which requires alignments after installation). I've found that oil changes, air filters, wiper blades can be done at the dealer for about the cost of parts (at least for my Ford and Toyota).

Now, as far as fixing repairs. Most mechanics, especially at dealerships, will toss parts at the problem. So knowing how to research the problem can help save you a boat load of money, even if you do none of the physical work yourself, since you can come in and ask for them to perform some specific fix. Additionally, lots of sensors are easy fixes if you can follow direction on YouTube/forums.