[Infinitum]

Well there's a number of things we can take into account here.

Most new users that create a shop will 100% be asked for a vouch copy to prove that the item they're selling is legitimate & working.

New users understand that and usually give a vouch copy to a trusted member or, even Oni if they'd like. https://sinister.ly/Thread-Vouch-Services

Another thing you could look at is if a member is pretty much only registering here to use the marketplace, given the fact that they need to make a post requirement to do so they could possibly spam the forum until they reach the required post.

A suggestion I would personally agree on would be a reputation requirement, this way they can't just spam the forum to get what they want as they'd get negative repped.

What's your thoughts on this?

The rep requirement is a good idea.
It'll probably permit to avoid a lot of scams (sarcasm here)

A lot of rep doesn't mean there will be less scams. If scammers are intelligent, they'll wait to get the required rep and will exit scam.