High transaction fees on sales. Typically at least 5% in US, not sure about Canada.
Also don't forget taxes, insurance, mortgage interest needs to get paid while house is in your hands. Most mortgages have lots of interest during the first few years.
Can you imagine if buyers and sellers only had to pay, say 1% each? The whole market would become a lot more liquid. But nah, brokers are essential and have your interests at heart.
This issue is frequently raised on HN. If estate agents only got 1% of sales, then could not even cover the cost of an office and (usually) a car and pay them self a middle class wage. Most people buy one or two houses in their entire life. It makes sense to have an expert to guide them through the process.
Im not sure about Canada, but Private individuals can list on redfin and Zillow in the US. MLS posting must be from a broker, but you can hire one flat rate upload your listing
Also don't forget taxes, insurance, mortgage interest needs to get paid while house is in your hands. Most mortgages have lots of interest during the first few years.