The compensation that a Sales Representative receives typically comes from the Seller's proceeds. In other words, there is no cost for a buyer to use a Sales Representative in a traditional Sales Representative/Buyer relationship. In a Buyer agency agreement, there may be some cost to the Buyer, but even these agreements are usually worded so that the Sales Representative is compensated from the Seller.
The sale of the severed portion of vacant land would not be subject to GST/HST since it is not capital property used primarily (more than 50%) in a business and it is not being sold in the course of a business. However, if the land was subdivided into more than two parts, sales of the severed portions are not exempt unless the portion is sold to a relative (or to a former spouse or common-law partner) for their personal use.