Tax reporting (Form 990) and Accounting (GAAP) for Auctions can be challenging. Below are good some articles and posts from around the web on the subject.
Nonprofit Auctions: A Complete Compliance Guide and Sample Forms
Charity Auctions: Reporting and Tax Rules
See this post on Nonprofit Accounting Basics
Tax Considerations in Charitable Auctions
See this article on Planned Giving Design Center
Accounting for Auction Items
See this article on graceknell.com