That warning is just telling you that they haven't updated their HTTPS security certificate. It doesnt' actually impact the security, just the certificate that Network Solutions or whomever keeps for them. It's sloppy bookkeeping on the site's part, and it's a bad sign.
I don't know that i'd buy from a site that isn't updated (copyright 2003), and one that makes it hard to figure out who they really are. This site is run by a Dodge/Jeep dealer in South Carolina, NOT by Mopar.