I realize this is proably achievable by jumping through a few hoops with respect to tier pricing or customization, however, a lot of stores offer bulk quantity discounts.  

I.e.

Buy 1 - 0% off
Buy 5-10 - 10% off
Buy 20 - 15% off

Making this an easy process for sellers I think we would be an attractive feature.  Food for thought.

Bill,

GrandNode has tier prices implemented. It should handles it I think.