I did this a few months ago and had no trouble.

There is no duty under $2500 if it was made in the USA and private sale. Bring a copy of the manual for the machine, or other proof it was manufactured in the USA.

I can't remember if it's the same thing for import as export (I'm Canadian), but for export I filled out form 7523.

Bring some kind of signed invoice or other proof of fair market value (a few recent archived Pinside sales couldn't hurt) and have a contact phone number in case the US border want to phone the seller to confirm.

Yep, it should be no problem whatsoever but I go over the border once a week for commercial purposes. I can tell you, from experience, it depends on the particular guard and his/her mood and how they react to you and your communication at that moment. They are trained to read PEOPLE and question PEOPLE and they will sometimes challenge you to see if what you're saying is accurate/your reaction to being made to feel uncomfortable.

The guard I had looked at the form, didn't even look in the trailer at the machine. But I've had other experiences where I've been hauled into US Customs and given the third degree. And yes, as per above don't have your guy mention a third party. Need to know basis and they don't need to know that. It could unnecessarily complicate things.

Just have your guy be prepared for the worst and if you sail through...awesome!

And Ketchup potato chips!