Yes, I am aware that the responsibilty for determining the tax liability is with the broker.
However, if the capital gain from the sale is UBTI, then it may make sense to not sell the preferred stock and instead withdraw it from the Roth IRA when I am 59 1/2. This allows the cost basis of the withdrawn preferred stock to be the value on the date of withdrawal, and avoids the UBTI in a Roth problem.
For many MLPs held in tax advantaged accounts it may make a lot more sense to withdraw them from those accounts into regular accounts rather than sell and pay the UBTI income tax penalty.
