Well is not a unique idea but it can be done with just a single person. Web developer, a smart contract and a marketing staff to get more people to use your ad site. Marketing might not be so easy, you can target the users of this forum but they will surely know where to do to post an AD here in the forum than on your website, the bitcoin job site is struggling still after years, yours would probably be going to same direction. Good luck finding a partner.
I share your opinion. It is not a unique idea and think that competition of the regular competitors is too huge to make this successful. And in my opinion there is another huge hurdle to take to make it fully working on crypto. Most (95%) of the crypto holders like crypto for its anonymity. But if you are going to offer a classified like craigslist customers need to share address, etc to receive the purchased good. And not a lot of them are willing to share their address or any other personal info.
I have invested in a couple small crypto related startups (no ICOs, hate them) but they all offered something unique, new, never seen. So for me as investor its a no go for me. Also you are looking for an investor. You will do marketing and website building. But only the investor is putting in money. So it would be obvious that the investor receives the biggest cut of the generated income and you as developer and marketeer only a small percentage for the work you have done.
I think if you are a good marketeer and web builder you better sell your services on fiver, freelancer, etc instead of launching your own project.... unless you have a unique and never seen and attractive idea