Texas most definitely has some strange rules on taxes with regards to
aircraft (and a few other things as well). The "use tax," as opposed
to a sales tax, is just another name for a sales tax on something
bought out of state (or in-state where no sales tax was collected).
It's basically the difference between what would have been due on an
in-state purchase and what was paid out of state. [For example, if I
buy a car out of state and pay 5% sales tax there, then bring it into
Texas, I owe an additional 3 1/2% "use tax."]

In the best case, there is no tax on "occassional sales" between
private individuals within the state. That should be his starting
arguement. However, if he bought it from the factory, then that will
not stand. It's a business-related sale. He can argue the "no tax on
internet sales" issue. That's the next best shot. However, there are
two problems there for anyone living in Texas.

First, a line item was added about 10 years ago that specifically
presumes aircraft to be business property and therefore tax is due. He
will have to prove that he has absolutely no business use for the
plane. [Interestingly, the line item to the tax code doesn't even
allow for proving otherwise. It says if you own your own plane, you
must be using it for business. However, they have no desire to see
anyone fight this in court and will back down if you can prove
otherwise.]

The second problem may or may not exist for any particular individual:
If the individual has any tax certificate in their own name, then any
purchase they make may be deemed by the state to be a business purchase
and tax is due. This means that in theory, since I own a small
business and hold an assigned business tax number in my name (DBA),
that if I give my neighbor $10 for an old tennis racket, it's a
business purchase and I owe use tax. In practice, of course, they only
find out about items during an audit, or big ticket items such as
aircraft and automobiles. Their computer program looks each year at
all autos and aircraft that are newly registered within the state, and
a card stating "you owe tax" will be sent (whether you eventually wind
up owing the tax or not).