If the USPS accepted mail that required a signature of the addressee, then they failed in their duty to obtain it and it lead to in-fact
no notification to the addressee that personal property was to be sold at auction, there is a question of whether notice was served or not. And while OP would have no claim against the USPS he/she may very well have a claim against the storage facility for failure to assure that actual notice of the pending sale was delivered. It is not sufficient to mail a letter and wash your hands of the responsibility to notify an owner of the pending action. The Federal courts hold this to be so.
There are plenty of Federal cases for you to read if you care to educate yourself.
https://scholar.google.com/scholar?h...ivered++&btnG=
One such case is
JONES v.FLOWERS et al. where a tax sale of a property was involved. The court said that when taking a person's property requires notification, merely sending a letter may not meet the statuary requirement that the owner was actually served notice.