GR translation: standing in for the rest of the tip

[Mike Jing]

P325.6-19   But I’m S.O.S., yes well actually this goes on, verse after
verse, for quite some time. In its complete version it represents a pretty
fair renunciation of the things of the world. The trouble with it is that
by Gödel’s Theorem there is bound to be some item around that one has
omitted from the list, and such an item is not easy to think of off the top
of one’s head, so that what one does most likely is go back over the whole
thing, meantime correcting mistakes and inevitable repetitions, and putting
in new items that will surely have occurred to one, and— well, it’s easy to
see that the “suicide” of the title might have to be postponed indefinitely!
          Conversations between Ombindi and Enzian these days are thus a
series of commercial messages, with Enzian not so much mark as unwilling
shill, standing in for the rest of the tip, who may be listening and maybe
not.

What is the meaning of the word "tip" here?  And who is the "who" in "who
may be listening and maybe not"?
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