We currently use a tax exemption plugin from IgniteWoo. I hate them and would much rather buy a plugin from you, so here's what their plugin does. Maybe you can re-create it and make a bunch of money.
It's pretty simple, really. The IgniteWoo plugin gives us three extra fields on a user account:
1. Tax Exempt
2. Tax Exempt ID
3. Tax Exempt Expiration Date
Those three fields are the conditions for removing the sales tax on the front end. If the user is marked as tax exempt and has a tax exempt ID which has not expired yet then when logged in the user gets to check out without having sales tax applied.
We've seen some solutions for allowing the customer to enter this information on an order-by-order basis on the front end, but in our opinion this invites abuse by people who simply don't want to pay sales tax.
If you replicated this incredibly simple plugin we would definitely buy it just to get away from IgniteWoo. If you figured out a way to improve on it that would be even better.
FYI - IgniteWoo didn't have this plugin either until we asked them to create it. They had it done in less than a week.