Aileen Wuornos, a notoriously uncooperative person, gave writer and director Patty Jenkins access to hundreds of letters she had written and received in order to gain insight into Aileen's life.
The real Aileen Wuornos actually used a storage facility, like the one seen after she leaves Selby's house the morning after they first met. Although in the movie it's shown only once (and before the killing spree), in real-life she did keep some of her victims' belongings there. She sold several of the other items to pawn shops, which eventually led to her thumbprint identification and subsequent arrest.
Selby is based on Tyria Moore, Aileen Wuornos' real-life girlfriend at the time of the murders. Selby is, for all intents and purposes, a fictional character. Moore was a notoriously reclusive person and didn't give out many details of her personal life and asked not to have her name and likeness used in the film, to which director Patty Jenkins agreed. The filmmakers had to use a lot of imagination when writing Selby.
When Lee is first shown interacting with a john in the movie, she shows him a photo of two young children, saying they are her children and she needs some cash to get to them. According to Aileen Wuornos: The Selling of a Serial Killer (1992), the real Aileen Wuornos actually used this method to lure her johns into paying for sexual favors because her physical appearance was starting to degrade quickly due to her wild lifestyle, so it was becoming difficult for her to earn money as a prostitute. She showed photos of her adoptive sister's children, claiming they were her own children. Wuornos' grandparents legally adopted her as an infant after her biological mother abandoned her, which means that her adoptive sister was actually her aunt.