The assumption you have is they are simply a market matcher but they're offering more than that.
Their incentive is to get the highest possible sale price in the shortest amount of time and a good reputation. That high price has to also be clearable meaning it cannot be just up in the air and seemingly arbitrary (it can cost either time or reputation).
Without them there is much more information asymmetry. Using an online version you basically have access to the same information you would as with them but without the human element to judge through nor their experience on offer.
The incentive for the agent is actually highest cash in down payment because they don't want to deal with the bank to get their money plus they get to keep the down payment if the buyer pulls out.
> plus they get to keep the down payment if the buyer pulls out.
What? Where does this happen?
In many places, the law is forcing you to go through an accredited agent to make a deal.