"To sell out" is to exchange your talents for money. Some artistic people consider it beneath their dignity to do so. For example, a rock band who licenses their famous songs for use in TV commercials can be said to have "sold out."
I can also mean to make compromises of one's "artistic integrity" to increase one's commercial appeal. Like the hard rock band that records a ballad to try to appeal to a wider audience. Or the filmmaker who has to change his movie to make the film studio happy.
I don't know the context you cite, but "the kid" obviously thinks highly of himself and considers working in an office to be compromising his integrity.
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