How people buy or sell ip addresses

When I read the article about “Ip addresses is sold out”. I find some interesting sentence “...but some small organizations have free ip addresses and they sell it” and “MIT find in their organization 14 million free ip addresses and they sell 8 million”. My question is - how they keep ip addresses or sell it? In what structure? This is a file or what. What means they “have”? They have it in file? Certificate?