The issue is that the flat percentage incentive is not a valid one, there was a chapter in Freakonomics about that (I seem to remember it was about Chicago estates, but it could have been another city in the US).
The study determined that when a house was the property of the estate agent sold in a little more time at a noticeably higher price.
The idea if that your house sells (today) for (say) 100,000 units of value (pounds/dollars/whatever) the estate agent will get (still say) 5% of it, i.e. 5,000 "units" (also today).
If in a few more months time you can sell it for 120,000, the agent will get (a few months later) 1,000 "units" more, but he/she will have spent a lot more time visiting the property with prospective buyers, runnning ads on newspapers, whatever, so the incentive for the agent is to conclude the sale as soon as possible, even if at a (reasonably) lower price.
The proposal to correct the incentves was to have a flat percentage (like the mentioned 5%) until a certain amount and a much higher percentage (like - if I recall correctly 15% or maybe 20%) on the excess.
Of course it depends on the local current market, but estate agents have interest to push house price higher only until the property is an "easy sale", the sheer moment the house stays on the market for some time (this highly depends on the local market, it could be weeks or months) they will start saying something "Hmmm, maybe we valued a little too much, we (that means you) should discount it by 10%", this in the same 100,000 unit house means that you will get 10,000 unit less, while the agent would get 4,500 units instead of 5,000.