[pfortuny]

There are two different concerns here which I think your abstract setting is mixing, probably involuntarily:

a) Privacy and secrecy are part of normal personal relations. Any means of communication can be used to maintain secret conversations and this is something one should strive for because confidence and trust between individuals cannot exist in a never-private environment.

b) On the other hand, the need for society to regulate contracts is there. A contract is not (has never been, at least in the West) a secret affair: it is something which society recognizes and whose terms can only be enforced if the mutual agreement is in some sense "public" (i.e. if it can be naturally known by society). So secret contracts cannot be lawful and in this I agree with you that secrecy in "civil" matters makes no sense (you would need a "secret police" and/or "secret judges" to enforce contracts, that is a parallel society, which as mafias have proved time and again, is a problem, not a solution).

So privacy and secrecy are natural and necessary for interpersonal non-public (i.e. extra legal) relations. Not so for "public"/"civil"/"contractual"... ones.

Notice that buying and selling is a "contractual" relationship (i.e. a willful, free, interchange of benefits with possible adverse consequences. It is the adverse consequences that require the legal environment and their being "public"/"accountable for").