It's a U.S. issue, covered by §1445 of the Internal Revenue Code.
Art -
Unfortunately, the IRS interpretation of the exception you relied upon appears to be that the new owner must use the real property as a "home", under the common use of that term. Although it's unlikely that the IRS would press the issue with you, there is some possibility that the IRS could come after you for that withholding tax.
Further, that exception, even if it does apply, would require the purchaser to occupy the timeshare for each of the first two years. If the current year’s week has already been banked or if the seller is keeping this year's use that wouldn't be possible. Also, the unforeseen circumstances rule would apparently not allow the purchaser to change his/her mind and exchange or rent instead of occupying the week in most circumstances.
Thus, I would advise others to follow the intent of the law rather than trying to follow a very narrow and possibly faulty interpretation to try avoiding the withholding. Note that the purchaser is the person with the potential liability for the withholding tax!