If your business explicitly deals in user information:
> In order for processing to be lawful, personal data should be processed on the basis of the consent of the data subject concerned or some other legitimate basis
> ... these are the other legal bases:
> 1. Processing is necessary to satisfy a contract to which the data subject is a party.
In that business it's necessary to share the data which is being sold. It's part of the contract. As long as you comply with notification, listing 3rd parties, and other relevant points, you can require and sell that data.