What was the name of the man who was selling a...truck maybe? farm equipment? -- drove away with a stranger who wanted to buy it, and was never seen again? Was it ever solved?

I think Oliver Munson is the one you're referring to.

http://unsolvedmysteries.wikia.com/wiki/Oliver_Munson

Details: Oliver Munson was a school teacher and car enthusiast who purchased a car from Dennis Watson, a known car thief who sold stolen cars with faked papers. As the authorities were closing in on Watson, they asked Munson to testify for them, but he never had the chance to do so. Munson vanished before he could appear in court.

Results: Unsolved. In 1985, Oliver was declared legally dead, the victim of "presumptive homicide". His body has never been found. Watson was released from prison in 1999 and has never been charged in Oliver's case.

No, the man I'm thinking of is white. I remember the reenactment took place in a farm-type setting, and the man's son was a witness to his father driving away. I can't remember what he was going to see with the stranger, if it was a vehicle or some sort of property or what.

Probably not the one, but its close to what you're describing.

http://unsolvedmysteries.wikia.com/wiki/Dexter_Stefonek

You're thinking of the case of Leonard Dirickson, which is a pretty baffling one and sadly still unsolved:
http://unsolved.com/ajaxfiles/mis_lenny_dirickson.htm

No, the man I'm thinking of is white. I remember the reenactment took place in a farm-type setting, and the man's son was a witness to his father driving away. I can't remember what he was going to see with the stranger, if it was a vehicle or some sort of property or what.

Yes, I second RobinW: Leonard Dirickson is almost definitely the segment you're describing. The man in the truck drove up to Dirickson's dairy farm one morning and supposedly wanted to see about a stud horse Dirickson owned. Dirickson's son Jared did not recognize that man as anyone he knew but did witness them both drive away.

That's it! Thanks so much!

So sad that it's never been solved.