I could be mistaken but I don't believe the seller pays the tax-the closing company who handles the sale actually is the one who should (automatically) withhold the tax from the final settlement and sends it to the IRS. The international seller can claim it back from the IRS by registering for a tax ID # and filing specific forms. Technically speaking I believe the buyer is responsible for withholding the tax so if for some reason a closing company goofed and didn't withhold it correctly the buyer could be responsible for paying the IRS the correct amount. In order to submit the necessary paperwork for the seller to claim the withheld tax back the buyer's social security # must be given. We were not comfortable sharing our social security # simply to buy a contract from a foreign seller (we did not finance so there was no reason to have to give it otherwise).
........I decided to actually google it and here is the info direct from the IRS website
FIRPTA Withholding
Withholding of Tax on Dispositions of United States Real Property Interests
The disposition of a U.S. real property interest by a foreign person (the transferor) is subject to the Foreign Investment in Real Property Tax Act of 1980 (FIRPTA) income tax withholding. FIRPTA authorized the United States to tax foreign persons on dispositions of U.S. real property interests. A disposition means “disposition” for any purpose of the Internal Revenue Code. This includes but is not limited to a sale or exchange, liquidation, redemption, gift, transfers, etc. Persons purchasing U.S. real property interests (transferees) from foreign persons, certain purchasers' agents, and settlement officers are required to withhold 10 percent of the amount realized on the disposition (special rules for foreign corporations). In most cases, the transferee/buyer is the withholding agent. If you are the transferee/buyer you must find out if the transferor is a foreign person. If the transferor is a foreign person and you fail to withhold, you may be held liable for the tax. For cases in which a U.S. business entity such as a corporation or partnership disposes of a U.S. real property interest, the business entity itself is the withholding agent.
That could be another reason too. It seems that it is much easier for Canadian sellers to get their documents notarized (more US notaries in Canada or they could be close to the US border) but in many countries US notaries are typically only found at US consulates. This can obviously slow the process as they have to obtain an appt to see the notary and possibly travel far to get to the consulate. Perhaps they have had enough complaints from buyers who weren't aware of the possible delay to decide to disclose upfront so people can't say they didn't realize?