Guy spots one of the popular girls in his class with an old man, she asks him to meet her the next day to clarify that she in fact was whoring herself out, and reveals she's doing it for money. From the "used goods" line we infer that she doesn't have much of a self esteem. He doesn't really give a shit about her an keeps it a secret.
She keeps telling him the two of them should fuck and pestering him about his crush, but he keeps putting up with her.
At some point people start thinking the two of them are going out, even the girl he likes, so he decides to tell her to stop that shit but at the same time it just so happened that she got the shit beat out of her, presumably by one of her clients. At that moment she decides to confess and tell him that she likes him only because he's the only one that's never been interested in him, that her upbringing was fucked up, and basically asks him to fuck her and go out with her. He pities her, and this makes him see her in a different light, but he rejects her anyways.
Some days later she's missing from class and it turns out she needed money for her dying mother, and when she passed away she moved away with a relative. He feels guilty.
Later he finds a letter with a phone number in his locker and decides to call her back rethinking his decision of not going out with her, but some other person answers the phone. He never sees her again.
In the end, it was all just him reminiscing and we see him coming back home and married to the girl he liked, who is now pregananant with his child.