Attorneys said Sparre, who was living in Georgia, responded to the online ad, and he and Pool text messaged each other for about a week until they finally met in Jacksonville.
Sparre told Pool he was visiting his grandmother, who had to go to St. Vincent's Medical Center, prosecutors said. Surveillance video shows him walking around the hospital.
Prosecutors said the two met at the hospital, but eventually went back to Pool's apartment.
Prosecutors said Sparre not only killed Pool with her own kitchen knife, but stole some of her things before leaving and heading back to the hospital. They said he even sold her PlayStation to a pawn shop in Georgia.
Police said it was four days until a concerned friend went to check on Pool and found her dead in her bedroom.