Does HST apply if the property I'm buying through power of sale was previously used commercially?

Possibly, and this is genuinely a fact-specific question rather than a simple yes or no. Whether HST applies to a real property purchase under the federal Excise Tax Act generally depends on how the property was actually used and its classification for HST purposes, not simply on who happens to be selling it. A property with a history of commercial or investment use can be treated differently than a purely residential property, and the answer can also depend on things like whether the property was ever converted between uses, and the status of the parties involved in the sale.

Because a power of sale vendor is typically a lender rather than someone with detailed personal knowledge of the property's tax history, getting clear, reliable information about prior use to properly assess the HST question can be harder than it would be with a cooperative seller who lived there or ran the business themselves.

Given how technical and fact-dependent this area is, get advice from your lawyer or an accountant on the specific property's HST treatment before you close, rather than assuming either that HST does or does not apply.