You mean the nth user gets a refund when the (2n)th user buys? As n approaches +inf.0, the incentive approaches zero. So it's likely to encourage early adopters

You mean the nth user gets a refund when the (2n)th user buys? As n approaches +inf.0, the incentive approaches zero. So it's likely to encourage early adopters, and not latecomers. But usually (I believe) the early adopters need no extra motivation. So the question is whether (sales/2) in this scenario is greater than (sales) in the usual scenario. Would it be considered gambling? For the buyer, or the seller? :) It sounds a bit closer to MLM than to gambling ...