I likely do not have enough information for you to consider but you seem to have most issues quickly solved with your expertise so I will try.
I have Vbulletin 4.1.9 installed and just re-installed 4.2 Classifieds. Install did not produce any errors.
I am able to login to admin and update changes. My first test with user trying to login produces error:
After refreshing the user shows as logged in but when switching around site the classifieds becomes logged out but other sections will remain logged in. Can you advise where to look for incorrect setting that might cause this?
Also I wish to use this program as more a business listing and not for selling products. My hope is to allow rescue shelters and similar groups to create ad (listing) that can be found in program. Do you think this is achievable?
Thanks for you help,