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 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.
 He refers to the gentleman, right?
 “who must buy” is the non-restrictive clause for “the customer“
 “he” refers to “the customer” here.
 How is “not the thing he desires” related to other parts in the sentence (ignoring the parallel “but the thing...” part afterwards)?
Your answers to ,  and  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.