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 for nearly $US100,000.

The appearance of the brass key and an accompanying memorial plaque on the internet auction house offended Adelaide officials, with mayor Stephen Yarwood saying he was "exceptionally disappointed".

'Trying 2get2 bottom!' ... Cher has suggested her office 'fkd up'.

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

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