CarSalesman wrote:

Now, if another employee goes with me, does that change
anything with the FAA? (it clearly does not with the IRS).
Sometimes another employee of the same corporation will
attend a meeting with me. Sometimes I will take a couple
people with me to an auction. They drive cars back.

Still no problem for the IRS, the business purpose is there, but
does taking another passenger along violate the "incidental"
part of 61.113(b)?

I don't think it does, provided:
1) you personally are not receiving compensation for the
flight
2) taking the flight is "optional" for the employees, they
could drive or fly commercial if they wish but choose
to come with you

The reason I think this is true is that the flight is still
"incidental" to the business ie, your business is buying and
selling cars, not buying and selling airplanes or using airplanes
to photograph cars from the air or what-have-you.

Cheers,
Sydney