As a man who has dedicated his career to real estate and Century 21, I may appear a tad biased when I say don't underestimate the power of a real estate agent, but it is in fact very true. For anyone invested in property, it is actually sound advice to never take for granted the influence a real estate agent has in a property transaction. Unless you are committed to selling or buying solo, in the majority of cases, the real estate agent is likely to be the most powerful person in the property transaction. For this reason, it is the agent who can make or break a deal.
If a real estate agent has been engaged, they are generally the only person who deals with both the buyer and seller during the negotiations to purchase a property, and this is why when you're purchasing a property it's imperative that you deal with the listing agent. This is the agent who has signed up the seller, and they have the closest relationship to them. They should also have their trust which is incredibly important if you are on the other side of the negotiations and you want their home.
Like any business, there can be office politics within real estate agencies, so it's important that you establish good relationships with the key people involved in your negotiations and transactions. Most times the listing agent will be the only one dealing with the vendor so other potential buyers still need to go through this agent. If you have an interest in a particular property, it's worthwhile finding out who the listing agent is and dealing directly with them.