This is a play on words. A "hard sell" is the use of aggressive salesmanship or advertising, which someone on Wall Street would use when conducting business. However, "sell" sounds like "cell" which is a room where a prisoner is locked up. Here, a "hard cell" refers to the uncomfortable prison where the Wall Street wizard is forced to stay.
Nearing the end of his sentence , the convicted Wall Street wizard , feeling both oddly safe in his spartan confinement but not quite penitent for his overzealous trading misdeeds , feared and anticipated his release , nightly dreaming of his long cherished hard sell .