In "Ex- Basketball Player," Flick's name represents an example of onomatopoeia in a couple of ways.
One way in which the main character's name has an onomatopoetic quality to it is in relation to his talents. "Flick" is associated with basketball, itself. The way the ball leaves a shooter's hand involves a "flick" of the wrist. To dribble a basketball effectively, one has to flick their wrist continually. Given how successful Flick was on the court, there was an effectively "flicking" of the wrist as he shot the ball. The narrator communicates this with, "He bucketed three hundred ninety points, /A county record still." It is also conveyed in how Flick still dribbles inner tubes "as a gag." The poem indicates how the "ball loved Flick." It is in these ways where an onomatopoetic quality to the poem's main character and what he loved to do exists.
I think that "Flick" has a specific quality to it because it sounds very quick. Simply saying, "Flick," is an exercise in rapidity. The name sounds over almost before it begins. This describes Flick's fleeting fame. His success has now passed, having flickered away. The time where he lit the home gym up with his exploits are long gone. In what he does now and how he looks at the world, his past successes seem like a "flick" in comparison. In this way, onomatopoeia can be found in his name and the thematic connections within it.