Thanks for your quick answer. If custom fields are not available, is there a global ID for each listing that I can grab using PHP and link to a custom form with the additional data I need to keep?
Another question that arose is the ability to pre-append weblinks so that clicks can be part analytics? Similar to what Google does for links on searches.
It is valuable to be able to show to advertisers the number of click-thrus to support the price of the listing.