I wasn't able to find direct information on Cox's history as a car salesman - but did find a couple of letters he wrote to the News-Leader regarding his whereabouts at the time of the disappearances. If I remember correctly, he first denied being in the area, but then changed his story - a couple of times. Seems he has a history of being charged with abducting people or holding them at gunpoint. He also has a history of getting caught, so perhaps not the best at planning - and this case definitely had some planning.
Seems it would have been fairly easy for LE to determine who, if anyone, Sherrill was dating at the time. She worked in an environment that is rich with swapping stories and for customers and co-workers to get to know you, who you're dating, hobbies. Seems like the salon owner or some of her fellow stylists would have known - unless Sherrill was keeping it a secret and what would motivate that?
Liz - thanks for the link on vidoq. I visited the homepage - you're right, it does seem that they require LE be the ones to submit a case. I wonder if Court TV or A&E has ever been contacted. Although, if they're smart, the producers are cruising these sleuthing communities for ideas on an ongoing basis. Also, I think they like to have closure to talk about and we've got none of that - yet.