Like a restaurant though, if they were bad at their job they wouldn't attract new clients. Under a HN analogy if they screwed clients they'd have a 0 rating and lose future clients.

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.