I think you are doing the right thing bo going for only 2 winners in each category as bigger prizes should bring in more volume as seen on axn.
One potential change i would like to suggest is to enforce a single wallet can win in only one category per round. E.g. single wallet can only win one prize in either sellers or buyers category in same cycle but not in both - this will prevent 2 whale wallets simply trading different NFTs between them and taking top spot in both buyers and sellers. I’m fine even if this sint enforced, but i think it would improve participation.