In the absence of any real ransom request directly following the abduction, one would logically assume that ES was abducted for a particular reason.
In my opinion that reason was sexual in nature. It was based on her nubile sexuality, her incredibly beautiful appearance, and youthful sensuality.
Whoever the abductor(s) is (are) he was only the fence or middleman in the operation. I suspect the "buyer" sought such a girl for child pornography and/or rape. Having grossly miscalculated the national attention her disappearance would bring and the intensive manhunt conducted to find the perpetrators, they most likely used her for their own personal vices and then killed her. It is highly unlikely that her body will ever be recovered.
Ten years from now reporters will be seeking Ed Smart out to review the abduction of his daughter on the anniversary of her disappearance. Most likely, he will have nothing new report.