You’ll need information on the purchaser of your property and the estimated gain or loss on the sale. Note: This is why you have to wait until you are under contract—only then do you have the purchaser information and sales price for your property.
To obtain the reduced withholding, the Form 8288-B must include the following information, along with supporting documents and calculations:
Name, address, and SSN of the purchaser
Name, address, and ID of the withholding agent
Details on the contract price, property type, and expected sale date
Information on the property itself
Confirmation that you have filed past US tax returns for the property, if needed (for example, if it was a rental property)
Calculation of the gain or loss on sale (so you need information on the original purchase price, capital improvements, and rental depreciation)
The reasons you would qualify for reduced withholding