You could issue sales receipts to the donors who purchased auction items with cash.  However, once again you need to make sure to also include the information about the goods they received in exchange for the donation (see below).  For instance, if they purchased a $50 gift certificate for $35 you should include a note at the bottom of the sales receipt stating that the donor received a $50 gift certificate in return for their $35 donation.

IRS Publication 1771has more information, but I copied the pertinent parts below:

Written Acknowledgment

Requirement

A donor cannot claim a tax deduction for any single contribution of $250 or more unless the donor obtains a contemporaneous, written acknowledgment of the contribution from the recipient organization. An organization that does not acknowledge a contribution incurs no penalty; but, without a written acknowledgment, the donor cannot claim the tax deduction. Although it is a donor’s responsibility to obtain a written acknowledgment, an organization can assist a donor by providing a timely, written statement containing the following information:

1. name of organization
2. amount of cash contribution
3. description (but not the value) of non-cash contribution
4. statement that no goods or services were provided by the organization in return for the contribution, if that was the case
5. description and good faith estimate of the value of goods or services, if any, that an organization provided in return for the contribution
6. statement that goods or services, if any, that an organization provided in return for the contribution consisted entirely of intangible religious benefits (described later in this publication), if that was the case

Regardless of what you send to your donors, you should enter sales receipts in QuickBooks for each donation so you can properly account for it.

Start by creating a bank account called something like “clearing account” (Lists > Chart of Accounts).  Then create a two-sided service item called something like “in-kind donation” (Lists > Item List).  You can make an item two-sided by checking the box next to This service is used in assemblies…  Lastly, uncheck the box next to Use Undeposited Funds as a default deposit to account (Edit > Preferences > Payments > Company Preferences).

When you receive an auction item, enter a sales receipt (Customers > Enter Sales Receipts) using the in-kind donation item in the Item field, an estimate of market value, and the clearing account in the Deposit To field.  Then enter a check (Banking > Write Checks) using the clearing account in the Bank Account field, the in-kind donation item in the Items tab (to the right of the Expenses tab), and the auction item donor in the Customer:job field.  Do not issue sales receipts to in-kind donors unless you remove estimated market value is first and add all the other required written acknowledgment information (see above).

When you sell an auction item, enter a sales receipt using your normal cash donation item, the actual cash received, and the undeposited funds account in the Deposit To field.  You can issue this sales receipt to the donor who purchased the auction item as long as you also add the other required written acknowledgment information, most importantly what was received in return for the donation (see above).

At the end, you should have an in-kind donation offset by an in-kind expense from the auction donor and a cash donation from the auction purchaser, with a net impact of increasing your net income by the cash received.