I’m not against payment requirement if there’s a guarantee that buyer will be able to use it towards their order or gets their money back under certain circumstances.
However, I think there are better and probably easier solutions. Reporting option should be fairly simple. If you get reported X amount of times then some kind of penalty will be given to your account.
Right now it’s a bit unfair that sellers with less than 90% rating won’t be able to make offers, but there’s no penalty for buyers to post requests. As long as you bypass the keyword filter you’re good to go.
Let’s face it, right now most of us don’t bother to write to CS about inappropriate requests. I think Fiverr is one of the few that doesn’t have this option. They can check UW or other sites if they can’t think of their own solution