IPv4 brokers act as an intermediary between sellers and buyers of IPv4 addresses. As you would expect, they are third-parties. It is their job to facilitate the trade of these address blocks between both parties in the transaction. Without them, you cannot think of a thriving IPv4 secondary market. After all, brokers are the ones who make the buying and selling of IPv4 addresses easier. To serve this duty, they enact many different roles likely only they can. Lots of existing brokers profit from this market while helping buyers and sellers meet their requirements. Here is an infographic that gives you a glimpse of what these professionals do.