If you are talking about selling or buying a property through an indivdual (not an agent):
Sellers will never be able to do the marketing and have the consumer reach that a full time, professional Realtor will have. Seller may sell at a price without commission but do they really net more money?
Buyers will not have the satisfaction of having a full time professional- knowledgeable in the market and transactions, to advise them.
"For sale by owners" sellers may let a buyer work with an agent for a commission.