I suppose the author could sell from their own website using a code, thereby registering the sale. They could also sell from e.g ebay/amazon etc via the same method: generating a code.

So long as sales are via the publishers website, they can absolutely defraud the author, but such is the case for the sale of any digital good.

I guess you could setup a "registration" scheme (e.g. to sub to errata) whereby readers need to supply their purchase code, which would reveal extra/double sells.