You tokenize each sale, just as e-junkie does, and only email the unique download link to the customer once the affiliate has completed payment, to you, of the wholesale price.

The automated chain would be:

1. Customer pays $27 to the affiliate, including his email address.

2. Upon receipt of the $27, the affiliate automatically pays $17.82 to you, including the buyer's email address (the system should ensure that the seller cannot simply not perform this step - there can be no problem of insufficient funds because, of course, the sellers PayPal account has just received in excess of the amount he must now forward to you. HOWEVER, in theory, if the seller somehow circumvents the automated system, well, HE has defrauded the customer, it is not legally your problem any more that Guchi is responsible for the guys selling fake Gucci handbags in the backstreets of Palermo).

3. Upon receipt of the $17.82, the system emails the unique download link to the customer.

Any later dispute is then the responsibility of the affiliate, removing the incentive for him to make false purchases.

I know that PayPal has been doing interesting things with their API but I have no idea how much of what I'm proposing might already be baked into their system. Either way, it would clearly remove a major pain point that has seriously disrupted (ended?) your business and, I'm guessing, affected everyone who sells digital products via affiliates (lots of people).

It's clearly an opportunity.