On average about 10% of homes in the US are sold without a Realtor.
The National Association of Realtors (NAR) reports that 5% of the time the seller knew the buyer, 5% of the time they did not know the buyer, and 86% of the time they eventually listed with a Realtor.
When a home is sold without professional representation, it sells on average for 16% LESS.
Homes are bought by a buyer represented by a Realtor about 90% of the time.
I do not know that ratio of homes sold without professional assistance for given geographic areas.