CRA acknowledges that a residential exemption certificate generally protects a purchaser from GST even where the vendor is non-resident

ETA s. 194 provides that where a vendor erroneously states or certifies in writing that it is making an exempt supply of residential real estate then, unless the purchaser knows or ought to have known that the supply is not exempt supply, the purchase price is deemed to be inclusive of HST or GST, so that the purchaser cannot be assessed for that tax.  CRA has acknowledged that this provision applies even were the vendor is a non-resident, so that its only recourse is to go after the non-resident for the tax.

Neal Armstrong.  Summary of CBAO 2014 Roundtable, Q. 25 under ETA s. 194.