I believe we have needed something like this for a long time. As @frank_d states, the seller needs to have "some skin in the game." In my category of voice over, we have over 25,000 sellers now which exploded after covid struck. Unfortunately many are people with no experience, and just the microphone built into their laptop, who figure, since its free to join, why not?
Fiverr could easily charge $250 per month to sell on the platform, then on a sliding scale, reduce that against the seller's billing. The more you bill, the less you pay for the monthly subscription, even eliminate it at some dollar figure. All the while still taking the 20%.
It would most certainly eliminate the sellers who are not willing to invest in their business. That raises the quality bar for buyers, and the revenue opportunities for quality sellers at the same time.