He might have saved $18000 commission, but he could've sold for $20000 more by selling through agents. (房市)
A recent real world example: Listing agent is a friend of the seller and agreed to not charge buyer's agent commission. So it's like a private sale + listing agent commission. There're over 50 showings. All but one buyer acted through an agent. There're multiple offers. The lone private buyer did make an offer. However, his offer is a lot less than the highest offer. Had the seller sold to this person, he indeed would have saved the 2.25% commission, but he would have made $13000 less overall.
I know at least 2 houses which failed to sell privately and later sold through agents. There're many issues with private sale.
The biggest one is lack of advertising. Most buyers search on MLS only and you'd miss probably 90% of the market. Even if a buyer looks outside MLS, he may not be able to find your house. There're many private sale operators out there. Most have really bad web interfaces. Earlier this year, I knew a house was for private sale but I couldn't find it at any of the private sale web sites I know. Later the owner hired an agent and lowered price.
Another problem is that people who look for private sales usually want good deals. They're likely to pay less than the general buying public, even beyond his share of the commission savings.
The biggest commission expense is for the buyer's agent. In theory, private sale should only save you the listing agent's commission which is probably only $6000 out of the $18000 total. If you refuse to deal with buyers who have agents, then you'd give up more than 90% of the market. I once checked 100 sales on MLS, 95 of them had a buyer's agent.