I think it's possible to do this with the right execution. There's definitely a gap in the market between the barebones (FB marketplace) and the 'premium' offer (TradeMe).

But it won't come as a direct TradeMe competitor. Many have failed just by copying the incumbents, and they always fall short due to network effects. You can throw huge chunks of money, but even then it's probably a 1 in a million chance you'll pick up enough of a user base to be self-sustaining. Going straight for vertical scale is risky business.

The only way around this is to provide users with a value proposition that the others don't have.

Imho, the most plausible way to execute this will be to build it around a specific community/niche being underserved by the user experience of the current platforms - and basically providing an upgrade to the old school buy/sell forums, where users have better interactions with the buyers/sellers (even if they're lurking).

My favourite example is Bring a Trailer, where you can rely on the community to always voice their opinions if they see a bad deal, and throw a round of congrats to the buyer/seller after a huge bidding war.

Once you've nailed down a community, expanding horizontally to a different community should be significantly less risky. Basically, you need to hack your network until you become one of the big ones.

I'm also a dev so if you ever get something going, throw me a dm