Your bank is serving as the escrow agent, so the CIP Q & A's indicate your customer is the person opening the account; i.e. the seller who leaves the contract for deed with you. As you note, you need the buyer's name, address and SSN for information reporting anyway, but you are not obligated to verify his or her identity.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.