I'm not sure where you live, but in the US, the standard commission for a registered broker selling a house is six percent of the selling price of the house, not a flat fee. But yes, they do not get to keep that six percent in perpetuity.
Any real estate agent in the UK offering that would get laughed out if town - 2% is common, 1% is realistically negotiatable
Sometimes a FSBO seller will offer to pay a buyer's agent 2.5% or so, unless they don't want to work with agents at all.
It's a waste of money IMHO, but some people like to have an agent involved. I've had far better luck selling property on my own than using agents though.