GST/HST Info Sheet GI-010
This info sheet explains how the goods and services/harmonized sales tax (GST/HST) applies to goods sold by auctioneers. It also explains how the tax applies to services provided by auctioneers. The information applies only to auctioneers who are registered or required to register for GST/HST purposes.
An auctioneer who sells goods by auction on behalf of a person who owns the goods (i.e., the "owner") is considered to have sold the goods to the purchaser. Furthermore, the sale of the goods by the auctioneer is either taxable or zero-rated even if the owner would not have been required to charge tax had the owner sold the goods directly to the purchaser. The auctioneer—not the owner—is responsible for charging and accounting for the tax on the sale. Of course, if the sale of the goods is zero-rated, such as a sale of cattle, the auctioneer does not account for any tax.
More details here on CRA website