Several years ago the Pittsburgh Press ran a series of articles about people that had their money taken simply because they were caught in airports with large sums of money. I remember one guy who had a nursery or landscaping business that was headed for a southern state to buy stuff.
He lost over $7,000 that was siezed for no reason other than he had it on him at the airport.
If you know the cost of something, you can always get a cashiers check. If you're not sure how much you're going to end up spending it gets to be a problem if you're planning on bringing your purchase or purchases back with you.
If you go to a construction equipment auction and you don't have a bank letter of credit, you better have cash. Lots of equipment goes for way over $10,000.
I know of one case where the drug dog alerted on a guy in the airport. When he was questioned, he didn't have drugs on him per se. But he had thousands of dollars that had enough cocaine on them that he was charged with possession.
In a lot of these "horror" stories, pertinent facts are left out. As for your machinery auction situation...what about traveler's checks? Safer than walking around with cash. Just a thought.