A corporation owns a wormhole system by living in it. They put up citadels or POSses, and bring in enough people and firepower to discourage other people from putting up citadels and/or taking out yours.
If you find an empty wormhole, you can sell it by logging off a scanning alt inside. Once you’ve found a buyer, he scans his way out and transfers the necessary bookmarks. Often there is a middleman involved trusted by both sides of the transaction that holds the buyer’s cash until it is verified that the bookmarks are valid and the hole is truly empty.
Alternatively, your corp might already be living in a wormhole but wants to move out. In that case you might sell the wormhole including all the infrastructure (citadels, POCOs, etc…) and save you and the buyer the hassle of taking down and putting up everything.