Chapter 1 - Selling What You Can't See
The author suggest selling a service is difficult not merely because it can't be seen, but it's absence generally goes unnoticed. We know what did happen and what is happening, but we are often not aware of what didn't happen and what isn't happening. And while it is possible to describe a world in which it is happening, there's nothing to base that vision upon.
With physical goods, you can at least show the client a demo, and get them to imagine the presence of the item in their office. When you're selling a contract to maintain the office computers, there's not really a good way to show them what it looks like - and even worse, there's no reason they have to expect they will need it when nothing is presently broken.
Warranties and service contracts are particularly troublesome because there is an element of gambling: if the item doesn't break the money you spent on a warranty was a waste. And when you are selling them the item, you can't be too emphatic about the warranty because it implies the item is poorly made and will likely break, making them less likely to buy at all. Insurance is similar - you're not even selling the item, but selling restoration in case something unfortunate happens (your car is damaged, you are injured, your house burns down). Even those who recognize such things are possible aren't happy talking about them, and would rather pretend the risks are not real than to be prepared for them.
Selling services that provide a proactive benefit (rather than address a problem that may not arise) are also problematic. If you're trying to sell an accounting service to a company, chances they already have an accountant who is already being paid to do the work, or maybe another service that is already doing it. If they are already satisfied, they may not be very attentive to your suggestion that your solution is better.
In essence, you're asking them to share in your vision of their future, and imagine the possibilities. Meanwhile they're thinking you're just trying to sell them something they do not need.
Selling a Promise
In essence, every act of selling involves selling a promise. Even when you are selling a physical goods, the prospect's willingness to buy it is based on their faith that it will do what it is supposed to do, as long as it is supposed to do it. A car must run, a vacuum cleaner must clean the floor, and an umbrella must keep them dry.
But for physical goods, they can handle them, kick the tires, and assess whether it is serviceable and sturdy. For service, there is nothing you can give them to assess and inspect to help them envision the benefits they will receive in the future. There is no evidence that, when the time comes, you will do what you promised.
Selling Trust
The value of a promise, in turn, is established by trust that the promise will be fulfilled. Most people have been taken in by others who make promises they don't keep, and you must convince them that you're not one of those types.
The author gives the example of ordering a restaurant meal - essentially that you expect that the meal you're getting is worth the price you saw on the menu when you ordered it. You may be happier with a cheeseburger in a greasy diner in a small town than you are with the cuisine in a swanky Manhattan restaurant - not because of the actually quality of the meal, but the degree to which what you were served met your expectations.
(EN: The author is skirting a very significant point, but doesn't quite get around to confronting it directly. Satisfaction is more often linked to the expectations of the customer, not the promises made by the salesman. You cannot argue someone into being satisfied after the fact.)
And so, when you sell a service you are selling a promise, and when you're selling a promise your selling your own trustworthiness. That is true regardless of what the service in question happens to be.