The re-seller certificate functionality has been improved with the following changes on the admin tab of user accounts. This new functionality is optional, so if you would prefer to continue using the current system, just let me know.
After you enter a tax info (certificate information) value you will have two more fields, the expiration date and the rate. The rate is the rate at which future invoices will be calculated for that user. The default rate is 0.00 and the default expiration date is one year from whenever you enter the information.
On the invoices, Re-sellers (Or farmers or whatever) will get a warning saying that they are responsible for letting you know if they do not want to apply their certificate to this invoice.
If there are buyers with tax certificates in an auction whose certificates have expired or are about to expire you will see a warning in admin letting you know so you can deal with that however you wish.
For users that already have tax certificate information stored you will need to manually update the expiration on their certificates before this new system will work for them because there was no way to determine what the proper default expiration date should be in advance.
Please let me know if you have any questions or you would like me to walk you through the changes.