Customers also tend to try to get the largest slice of the "better off." That's not special. Do you take less pay than you could get because you don't want to be greedy? Do you voluntarily pay more than the listed price when purchasing items? Of course not.
In a properly functioning market, the limiting factor is competition. A business can't capture 99% of the "better off" because the guy down the street will undercut it by only capturing 98% and all the customers will go there. A customer likewise can't capture 99% because the business will decide it's not worthwhile and sell to customers who capture less. Somewhere in the middle is found equilibrium.
Bag-holding is different, it means there's some sort of trickery and the other side doesn't actually benefit from the transaction. And this does happen, but it's not the norm. Again, go take a look at your local shops. Which ones are engaging in this?
There is a 'human' process 'somewhere in the middle' as you say.
I'm not anti-capitalist, I just acknowledge this as a fundamental part of the human process. It might even be the case the businessman's incentive don't fully align with the stockholder/owner's incentive, in many case the businessman can profit at the expense of the company by capturing higher commission with side-effects that the company won't understand.
A clever businessman will get the better end of that, while still leaving his customer better off than if he hadn't transacted at all, as many witness at the car dealership.
My point is that most business is in that middle. A typical business transaction ends with both sides getting a decent chunk of the value produced. Abusive transactions happen but they're not that common, certainly not the majority as the other comment said.