This a very good question!
CRA does not specify in any of their documentation about claiming GST/HST specifically on the real Estate Commissions to sell your old home. However, they do clearly state that GST/HST is deductible on certain moving expenses, and not deductible on others (such as Legal Fees and transfer fees for the new home).
The information found in the following CRA IT Bulletin is as follows:
4.24 As mentioned in ¶4.22f, a goods and services tax or a value-added tax paid on the purchase price of a new home is not eligible as a moving expense. However, such a tax paid in relation to other costs that give rise to a deduction for moving expenses would be eligible. Examples might be the goods and services tax (GST) or the harmonized sales tax (HST) paid on the cost of renting a moving truck or on legal fees in respect of purchasing a new residence.
CRA Interpretation Bulletin - Moving Expenses
As we have not been able to find anything that discusses the GST/HST paid on Real Estate Commissions, we suggest you contact CRA for confirmation of this. If we do find a concrete answer, we will be sure to paste the updated information here.