As dgillz said, Flexibility would be the best solution for this. Flexibility could add totals for the existing orders and the current order and prevent a buyer from purchasing more than their limit. The limits could be defined globally or by buyer. If you do not have the resources on hand to accomplish this for your client, we would be glad to help you with this. Please just send an email to scott@ nexosystems.com (Please remove space), if you are interested.
Scott Travis
NEXO Systems, Inc.