The US isn't selling anything here. The selling and the buying is between two foreign companies, P&O and DPW. Since the deal involves the logistics movement of goods though US ports-of-entry, it was vetted and approved by many US government agencies headed by the Commerce department. The only thing extraordinary here is what's being drummed up by the press.
To insert some levity into the matter... :)
We are "selling" access. Access is key. It's the concept and the access that the concept provides. The agreement is, in effect, "selling" access because the Port Deal is a negotiation involving exchange of assets. Call it a lease, call it favoritism, call it a gift, call it anything you want but it's still a situation wherein the U.S. has "sold" access and access to privileged information and other very significant resources.
It's also just easier for most of us to type out in comments than writing out the elaboration of the actual terms involved.