IRS auctions
Real Estate, cars/vehicles, and more

Treasury Executive Office of Assets Forfeiture (TEOAF) auctions
Real Estate
General Property
Car Auctions
More Car Auctions  - (Customs and Border Protection, US Department of Homeland Security) 

Online Auctions and Sales
Surplus, Seized, and Forfeited Property (includes browse by state)  – GSA
Real Estate and Merchandise – GovSales  (some online)
Fleet Vehicle Sales –  GSA 
U.S. Marshals Service (some online)

In-Person Auctions and Sales
Fleet Vehicle Sales – GSA
Real Estate and Merchandise – U.S. Marshals Service
State and Local Surplus Property Sales 
Books and Merchandise – U.S. Postal Service 
Real Estate – Small Business Administration

Real Estate
HomeSales.Gov
HUD Homes
U.S. Marshals Service
Small Business Administration

Listed by Agency – Includes Gift Shops
USA.gov – The U.S. Government’s Official Web Portal

Other Auctions:

http://www.govliquidation.com/
http://www.govdeals.com/eas/
http://www.liquidation.com/bu/index.html