1) the seller has not made more than two sales of taxable items (not
just
parts for an aircraft) subject to sales tax within a 12-month period,
and
the seller is not engaged in the business of selling taxable items (not
just
aircraft parts) and 2) neither the seller nor the buyer has been issued
a
sales tax permit or the equivalent by Texas or any other state.

Right... That first one gets you into the "occasional Sale" arena.
The second is that little nasty that I referred to, where anyone
holding a valid sales tax permit is presumed to be making a business
transaction, no matter what or why the purchase was made. There is a
little additional wording in there somewhere (used to be subsection h)
that says if it's an airplane, the it *IS* purchased for business use.

But, as is true for most folks, if you can get by #1 and #2, then no
tax is due. [First plane I ever bought, they sent me the request for
Use Tax a good 5 years after the purchase. To agree that it was
Occasional Sale (second part of the rules hadn't been written in back
then) they wanted to seller to sign this notarized affidavit. Well, he
was elderly then, and selling due to ill health, and moving...
Probably cost me almost as much to track him down and get it signed as
the tax would have been. G