He knows you don't mail questionnaires to people on the other side of the law. First, these aren't smooth operators. They're people who've never had anything and have no idea how to manage money. He'll pay cash for fancy suits and cars. The users are a captive market.
Dealers keep crack prices high with little market influence.
He's found that sex is a commodity here. Crack increases sexual interest, but it dampens virility. Hamid studies the sordid details of sexual behavior to see how AIDS moves through this nether world. He studies the mechanics of family dysfunction. He learns what role rebellion plays in driving kids into this life.