Need advice getting traffic to a new Classified Ads website

I'm looking to build a new Classified Ads website and I badly need some marketing advice.

It'll probably not kick Craigslist out of business anytime soon -- but that's not my target for now. Rather to make it work and get some momentum. That's where the trouble starts - traffic.

I have some good ideas about what needs to be done (features, platform). Development is not a problem for me, I'm a full stack developer with a lot of experience. The site is not ready but getting done isn't a big issue. And I'm not new to owning this kind of site, I've been in this area long time ago. I had classified ad sites and directories, years ago.

The biggest problem you're going to encounter though, with such a project, is gaining momentum.

Getting enough ads/users and a large quantity of traffic. I need both.

I've recently seen how one of the bigger platforms did it - they spent millions in a massive TV ad campaign. I don't have that kind of budget now.

In the past, I simply used SEO to get that traction. That doesn't work the same today, and although it'll bring traffic later, I cannot really count on it. Classified ads websites are kind of generic, and due to this they don't rank very well. Side note many users post short ads and this leads to a large amount of thinner content.

I have researched certain ideas, but with no luck so far. Paid ads are probably out of question as it'll be way too costly. Or am I missing some way to do it. Tried 7Search, low CPC but junk traffic.

I know Backpage for example is making phone calls to posters on other sites and requesting approval to freely repost their ad. They even called me several times. This is cumbersome and kind of expensive I guess.

So my question is:

What tools/solutions/approach do you think I can use to promote this kind of website? What channels to use? How?

I can't shake the feeling that there's something I'm missing here. Any suggestions will be appreciated, thanks!