Abstract: Among other things, a server controls access to private information about attributes of a unique item, during one or more stages of an online transaction for the unique item between a first online party and a second online party. The controlling of the access to the private information includes the server digitally storing (a) the private information, (b) a listing of the unique item for the online transaction received from the first online party, the listing including one or more constraints on access to the private information by the second online party at one or more stages of the online transaction, and (c) an indicator of a current stage of the one or more stages of the online sale transaction. The server allows or prevents access to the private information at one of the stages of the online transaction through a browser or an app installed on a device of the second online party and based on the current stage of the online transaction.