It seems that Marcellinus is a common friend or acquaintance of Seneca and Lucilius, and his character isn't the best. Seneca and Lucilius seem to be trying to do something to help him improve his character.
He doesn't want to go near Seneca for fear he will "hear the truth." And Seneca is fine with that! Seneca is of the opinion that "one must not talk to a man unless he is willing to listen." He compares this to talking to a deaf person - it's useless. Seneca is in the camp of teaching only those who are willing to listen and change. He won't waste his breath on someone unwilling to improve.
The other camp is the like the salesman, who takes a talk-to-everyone approach. They think words are free and by "[scattering] this advice by the handful ... It is impossible that one who tries often should not sometime succeed." Seneca does not approve of this approach. The thinking goes: if you are always talking and hit the mark a few times, then it cheapens your words.
To use an archer analogy - who would you prefer? The archer who takes 100 shots and kills a handful? Or an archer who hits consistently every time he fires?
Seneca also thinks teaching wisdom and living wisely is an art. And if you are not discriminating in your art, can it really be called art? Where is the intentional, rational choice if all you do is throw words and teachings mindlessly and indiscriminately?
I can see the appeal to both. If you are appealing to the masses, then taking an all-the-above approach and casting a wide net might gain some followers of philosophy. But the quality may be low. On the other hand, being prudent with teaching and appealing only to people who are going to take it seriously, has a higher success rate.
Back to Marcellinus specifically. He is so vigorous in his lack of living philosophically, that he is a danger even to those who would want to help him. Like a powerful person flailing in the water, he could pull his rescuer under water and two people drown instead of one.
But Seneca puts up with him and hopes to at least check and slow down Marcellinus' vices if not to turn him altogether to philosophy.
His quote from Epicurus: "I have never wished to cater to the crowd; for what I know, they do not approve, and what they approve, I do not know."
The only commentary I'll share on this quote is to share another quote (source):
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.