CR should upgrade to the latest IPS board (not vbulletin). I use it on several sites and there are a ton of useful features. You can tag people in posts, get notified if you get quoted, embeds are seamless, mobile styles, etc. Also change the search time to 3 seconds and enable 3-letter searches. If there are some server resource limitations to keeping things locked down then move to a better provider. If there is no money to buy an IPS license then pass the fucking hat around.
Technicals aside, most of the site should be publicly viewable. You can't link for sale threads to anyone because they need a CR account. The areas you WANT safe from prying eyes? Make the topic list viewable (when you enter a forum category) but you can't actually view the post you click on unless you log in.
I actually have plenty more ideas but that is a start.