Agents get paid a commission when a home sales.

In almost every case, the entire real estate commission is being paid by the seller.  Sellers need help to get their home sold so they hire a Listing Agent.  The seller agrees to pay that Listing Agent a percentage of the price of the home when it sales. 

When the Listing Agent uses the computerized MLS to advertise the home for sale, then the Listing Agent is agreeing to pay a Buyer's Agent part of the commission. 

Seller's can pick any agent to list the home and the sellers pay a pre-determined commission no matter which agent sales the home.

Buyer's can pick any agent to help them do the house hunting and they almost never have any out of pocket expenses for the services of their buyer's agent, because the seller has already agreed to pay their Listing Agent and the Listing Agent has already agreed to pay the Buyer's Agent.