Hi,
Your requirements look quite clear and straightforward to implement. Not a detailed study but I've checked the site and I suppose it has serious precautions to block automation, as expected from a classified website. Because of that, though it's not certain at the moment, we'll much likely need to use browser emulation with selenium visiting the site like a regular user does to stay out of their radar.
About the program features, there'll be a simple but clean UI where you can manage your templates by category/subcategory. Also a list of your currently active ads if you prefer. On settings page you may define the interval the tool should run or schedule your ads to be posted at a specific time.
Such automation projects are my specialty and I've implemented several tools like that. You've mentioned in the description that you want this to be implemented in .net. I'm not sure if you prefer it to be a desktop or web application. I should note that if you prefer the former, desktop app, the pc it's installed on must be on for the tool to run obviously.
Anyway, let me know if you have any questions or I'm missing anything here. All in all, I expect this to take around 10 days to implement, thanks for taking the time.