US singer Cher is "upset" and trying to get to the bottom of why a key to the city of Adelaide she was given in 1990 ended up on eBay, selling Thursday for nearly US$100,000.

The appearance of the brass key and an accompanying memorial plaque on the Internet auction house offended officials in the southern Australian city, with Adelaide mayor Stephen Yarwoodsaying he was "exceptionally disappointed."

"It must be very embarrassing for her if she is selling it because she needs the money," Yarwood said last weekend.

In a tweet, the ageing diva said she was mystified about it appearing on eBay.

"I'm upset 2 & trying 2get2 bottom! I Think my office fkd up?," she said on Twitter.

The key and engraved plaque was sold by a user called Authentic_Cher who claims to be the musician's new "personal consignor", saying the item was from her Malibu, California home.

The auction closed Thursday morning after 146 bids from people around the world with the successful buyer paying US$95,900.

Cher, 65, was awarded the key to the city in recognition of her performance at the 1990 Formula One Grand Prix.

The man who presented it to her, former mayor Steve Condous, said he was bitterly disappointed at the sale.

"If she didn't want to keep it she should have returned it back to the city," he said.

Cher has earned both Oscar glory in her acting career and Grammy glory in her musical career, selling over 100 million records.