Officials in Adelaide have voiced their disappointment at revelations that US singer Cher had apparently put her key to the city up for sale on internet auction house eBay.

The key and accompanying memorial plaque was being sold online by a user called Authentic_Cher who claims to be the musician's "personal consignor" selling her personal effects.

Cher.

It had attracted a top ask of US$81,100 by Monday morning, with less than two days of bidding to go.

But the move offended officials in the city of some 1.2 million people, with Adelaide mayor Stephen Yarwood saying he was "exceptionally disappointed."