i had trouble with this on some of the sites we developed on 3dcart. i believe that the 3dc javascript validation relies on some of the div/table classes. so if you remove a class, it can't validate and won't add to cart.
best help i can say is to start over with the default listing template, and then be careful when you customize to keep the classes.
you might be interested in one of my other posts here:
Reduce the bulk of unnecessary code!