It's been a while since I thought about this. Rather than listing off features sometimes I like to take a step back and think about it from a user or seller perspective. I like what you have down already. The two key workflows that I feel many classifieds miss are ease of use and discoverability.
From a Seller perspective:
1. Listing - single page listing if possible with fields that are gradually revealed (this simplifies the experience).
a. Start with title
b. Pick category and subcategories (or default)
c. Add description
d. Specify for sale/wanted
e. Add price (or free)
f. Other core but optional fields are attachments (multiple images, featured image, ability to reorder/remove), location (option to show saved location or specify Google mapped location), contact (option to show saved contact info or specify).
g. Other custom fields.
I like the idea of a basic templating system where you can group particular items in a category perhaps under a template with its own set of custom fields. For example a car category will have custom fields such as Make, Model, Vin, Mileage etc. So your custom fields should be either global or per category (one or more).
Very importantly put a Post New Classified button/shortcut in forums where classifieds are posted as threads and give option to disable posting of new threads in there.
Ad lifecycle management - as seller quickly mark item as sold, unlist item (soft delete), hard delete item
Add Multi-Currency Support i.e. default currency + user can optionally pick from other defined currencies.
Feedback/Rating System?
User Dashboard:
1. Seller/Buyer preferences - save your location, phone number, contact email
2. Seller/Buyer message board - private conversations filtered by item for sale (my ads) or by interested in (replying to item)
3. My ads, my alerts, my saved items, my search alerts (more below)
Discoverability:
Classifieds Page
1. Filter widgets/sidebar - geo fencing by distance from my current location or specified postal/zip code, city or state/province or country. Filter by for sale/wanted, price min-max, category/subcategory. Should be ajaxified for a seamless experience when filtering.
2. Alerts - subscribe to alerts for new items per category, sub category etc.
3. Watch lists - search term watch + alert (this will be a daily search that generates an alert and email)
4. Order by featured listings then normal listings
5. Add rotating carousel/wide/narrow widgets for latest ads so we can add it to the home page and other pages
6. Integrate with XF search also
7. SEO - Google microdata/markup for product would help also