Simple buying services are not obligated to work or provide the best domain deal for customers. Most often they simply wait for buyers to request a domain. Then they simply contact the domain owner and make your offer. Often you pay a fee no matter what.
Professional domain name broker/agents like Brannans.com are active and proactive. They research similar domains and recent domain sales to determine an approximate market value. Then they advise their client — either a domain buyer or domain seller — on the techniques to complete the domain transaction successfully — always in the client’s best interests. This often requires hours of research and effort and experience and a professional domain broker does not get paid unless the domain transaction is successful.