I don't mean to discourage your initiative in any way, but from a realistic perspective, this project isn't going anywhere unless you rethink the whole approach. You don't build the product and then go on finding customers; it's usually the customers first that you're helping to solve a crucial problem. The existing classified sites can just add an option to pay with Bitcoin. Or they could allow the classified posters to mention their crypto addresses, if applicable.
It is a unique resource where users can post ads to buy, sell, and exchange cryptocurrencies, as well as offer goods and services for crypto.
Not so unique, BitcoinTalk does that already. There should be a strong reason why an existing seller would want to use your forum over Bitcointalk. Reevaluate your ideas otherwise you'd just waste time building something people don't need.