We need to offer a sliding scale of discount based on product volume and product history.

We are selling a ticket for an event to a reseller who gets a discount depending upon how many they have sold at any given time for example:

10% off orders of at least 10 tickets
20% off the next 10 tickets (total sold 11+)
30% off the next 10 tickets (total sold 21+)

It's possible for a reseller to cross over discount brackets in one transaction in other words they may buy 15 tickets. Also a reseller's history needs to be taken into account so that the discount accrues.

etc, etc

I understand the group pricing module will allow us to add a user to a group that qualifies for a discount of x amount but that doesn't help us since the reseller may cross over in one transaction from one discount bracket to the next.

The Quantity Discount module doesn't help because it offers the discount on an isolated transaction rather than taking account of the resellers previous history of buying that product.

I hope I have made this clear! Can anyone offer a solution?

Many thanks.