The buyer cannot currently deduct the tax; it should be capitalized as part of the basis of the purchased property (along with the other costs of acquisition, e.g. legal fees, recording fees, and such.)
The so called mansion tax is really a sales tax. You may elect to take the sales tax deduction in lieu of your standard deduction. You would do this if the sales tax is higher than the standard deduction for your filing status. ... But transfer fees are not deductible.
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