How to under the "not the thing" part?

The original text:

A gentleman is, rather than does. He is interested in nothing in a professional way. He is allowed to cultivate hobbies, even eccentricities, but must not practise a vocation. He must know how to ride and shoot and cast a fly. He should have relatives in the army and navy and at least one connection in the diplomatic service. But there are weaknesses in the English gentleman’s ability to rule us today. He usually knows nothing of political economy and less about how foreign countries are governed. He does not respect learning and prefers ’sport ’. The problem set for society is not the virtues of the type so much as its adequacy for its function, and here grave difficulties arise. He refuses to consider sufficiently the wants of the customer, who must buy, not the thing he desires but the thing the English gentleman wants to sell.

My question are about the last sentence. I've numerated my questions as below.

He[1] refuses to consider sufficiently the wants of the customer, who must buy[2], not the thing [3] he[4] desires but the thing the English gentleman wants to sell.

[1] He refers to the gentleman, right?

[2] "who must buy" is the non-restrictive clause for "the customer"

[4] "he" refers to "the customer" here.

[3] How is "not the thing he desires" related to other parts in the sentence (ignoring the parallel "but the thing..." part afterwards)?

Solution 1:

Your answers to [1], [2] and [4] are correct.

Not A but B describes what the customer 'must buy' - not what he wants but what the 'gentleman' wants to sell to him.