The commission structure is negotiated by the seller and the listing agent and as a result leaves no responsibility to the buyer.
Today's commission rates generally are in the range of between 5 to 8 percent of the selling price. For the most part the normal rate id in the area of 6%.So if you the home were to sell for $200,000 at a 6& fee for professional services the amount due would be $12,000.
To take this a step further, this amount is split 50/50 between the listing agent and selling agent. Then each agent's broker gets a percentage. there's E & O Insurance, franchise fees, advertising, etc. etc. By the time each agent figures in their expenses, they would probably be lucky to take home $2,000. Oh, then there's the IRS.
Most people have no idea of the expense or time an agent invests in each transaction.