I only occassionally cross a state line for farm-related purposes. Do I need at US DOT number?

An exception in 49 CFR Part 390.3(f)(3) of the federal regulations states a person may make an occasional trip to transport personal property neither for compensation nor in the furtherance of a commercial enterprise. Farming is a commercial enterprise and trips in and out of state to purchase equipment, sell grain or buy fertilizer would require US DOT registration. Likewise, travel out of state to purchase antique tractors for restoration and resale would require US DOT registration as would being hired by an acquaintance to travel out of state to pick up a purchase.