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.
Charlize Theron won the Academy Award for playing Aileen Wuornos on 29th February 2004, Wuornos' birthday.