The most straightforward way of enforcing the exclusive sale model I can think of is to use reputation and reviews similar to how the online drug sales sites wo

The most straightforward way of enforcing the exclusive sale model I can think of is to use reputation and reviews similar to how the online drug sales sites work. Create a financial incentive for the seller to build a positive reputation and deliver on their products quality and promises (exclusivity in this case). If the seller creates a name for themselves and they can be trusted to sell information exclusively then that will add value to the information (buyers would pay extra to ensure it's exclusive). That model will incentivize sellers to hold up their end of the bargain and prevent duplicate sales.