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The general rules for nonresidents of the US is that only US source is taxable by the US. For general information on taxation of nonresidents of the US see Taxation of Nonresident Aliens
Real property sales are reported on 2008 Form 1040 (Schedule D)
and on 2008 Form 1040NR
Complete information is available from Pubilcation 519 Effectively Connected Income :
"Gains and losses from the sale or exchange of U.S. real property interests (whether or not they are capital assets) are taxed as if you are engaged in a trade or business in the United States. You must treat the gain or loss as effectively connected with that trade or business."
Gains realized by nonresidents on the sale of U.S. real property will be subject to a 10% withholding tax on the gross proceeds. Alternatively, a taxpayer may withhold 30% of the capital gains realized upon sale of the property. The 10% or 30% withholding are required to be made at the source.
I hope this helps for understanding the taxation of sale of real property in the United States by a nonresident.
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