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: Dos and Don’ts
See this post on Nonprofit Accounting Basics
Tax Considerations in Charitable Auctions
See this article on Planned Giving Design Center
Accounting for Auction Items
How to Account for Charity Auction Items – Part I
How to Account for Charity Auction Items – Part II