I agree and I didn't say I was going to copy it
My plan is to make similar add-on that helps create custom marketplace. Its just a general idea so far, not my first priority: store add-on and updating styles are first in todo list.
I think you will make lots of sales if you do and support it properly. This addon was actually pretty good in many ways and I did appreciate the features it offered.
As with most classifieds addons best source of ideas is not to invent the wheel but make it look like the other 'wheel'. Look at kijiji, craigslist and many other localized classifieds platforms. If you make a kijiji 'lite' with the basic features you find there it would be a hit.
Important features (some of which this addon has):
1. Dedicated ad page
2. Child forums for ads
3. Buyer/seller messaging shortcuts
4. Feedback system
5. Proper Ad lifecycle management i.e. open, close, delete ads
6. Custom ad fields that are configurable
7. User group permissions
8. Easy to use sidebar filters (location, category, keyword, type etc)
9. Good thumbnail and gallery handling
11. Ad expiry/auto cleanup and my ads shortcuts
If you get half of the above in your first addon I'm sure most of us former clients of this addon will migrate.