It’s a question I’ve been asked for 30 years. Back in the early 2000s, pre-internet, we’d stand and watch a sharp, used tractor or combine sell for $200,000. We’d say, “Can these prices go any higher?

Fancy new technology usually is the star at trade shows. But at the Farm Progress Show, the largest outdoor agriculture event in the U.S., used tractors are the big attraction. Farmers at the Boone, ...