Perhaps too simplistic, but as a start surely all you have to do as a seller is ask a potential selling agent how much commission they intend to stick on top of your desired price (I have a neighbour who is selling a house from abroad and who doesn’t even know how much the agents have the house on the market for, which I find unbelievable).
And, as a potential buyer, also ask the estate agent who wants to show you their properties how much commission they make on a sale (of course you don’t know if they’re telling the truth or not but you could ask for proof to put them on the spot).
Get it out into the open. Why are we all too embarrassed to discuss estate agents’ extortionate commissions anywhere except in the bar or on forums?!
An estate agent (big, inland) once dropped a document in a friend’s car unfortunately (for the agent) showing the agreed price to the vendor and the selling price on 10 houses. Commission to agent between 10 and 50%!!!