We hosted an event with a silent auction and I've been asked by the event coordinator to send thank you letters to the people who purchased the silent auction items as a courtesy. Do I just thank them and describe the item they purchased or do I list how much they paid for it? There were a couple items that sold above "fair market value" and I understand the amount above fair market value is tax deductible.

My assumption is I can list what they bought, what they paid for it, and what the tax deductible amount is (for some it will just be zero). Does anyone have a policy for this? I want to be sure to thank them appropriatly.

What do you think?

You do not need to tell them what the tax deductible amount is and I recommend that you do not.  Per IRS publication 1771 you only need to state what they paid and what benefits they got in return (description and value) it is up to them (or their tax advisor) to decide if there will be any tax deduction for themselves.