Consider offering a monetary incentive to get customers to buy.

This 'one nation' pension would be easy to explain and understand, giving everyone the same monetary incentive.

The top ten most visited website relies on user contribution - from creation to editing - for its content and gives no monetary incentive for this contribution.

Players may even place a bounty on another player's head, providing monetary incentive for bounty hunters.

Objections described here include the programmer's need to make a living, the issue of advertising/distributing free software, and the perceived need for monetary incentive.