The unnamed book seller is the paroled rapist of his then 6-year-old niece. Olivia Benson both arrested the seller and was at his parole hearing, which mandated him with regular chemical castration shots in exchange for getting out early, leading to his obesity and other medical struggles.
The seller was a suspect in the stalking of Benson and serial murders connected to her. He wanted to know if she was dead, then denied any relation and asserted sticking to his parole terms no matter how much he hated it. (SVU: "Wrath")