Humvees along with trucks, trailers, etc. have been and are available as military surplus. Usually, it is a flat rate purchase and not an auction. An acquaintance of mine has been buying surplus Humvees for years to rebuild then flip for a profit.
From his description, most Humvees are driven until they are junk then are sent to a central depot (Maine or New England I think he said) where they are stripped with the used parts used to rebuild other junkers back to standard or forwarded into the military logistics supply. Bottom line is that Humvees released for surplus sale right now are usually in pretty sad shape and take quite a few $$$ to get functional.
Here is a surplus government website to get you started:
http://www.govliquidation.com/vehicles.html