Savings bonds are sometimes sold as souvenirs or collectors' items. The sale doesn't affect the ownership of the savings bond, since by regulation, a savings bond is a registered security and ownership is non-transferable.
The United States Treasury still has a contractual relationship with the owner or co-owners named on the bond, not the person who bought the bond at auction. Because of this, the person buying it at auction can't cash it--he's just purchased a piece of paper showing a bond that still is the property of the owner or co-owners named on the bond. In some cases, the bond may be the property of the United States Treasury (if it was lost and has since been replaced).
Bottom line: it's not a good idea to buy a savings bond at an auction, because you do not acquire any title to the bond or have any ownership rights.