South Miami has been offered millions of dollars from developers who want to buy City Hall, knock it down and build residential and retail. While a cluster of residents disapprove of selling City Hall, commissioners and staff are negotiating its purchase price. Oxford Group Holdings was the second party in as many months to make an offer for the property at 6130 Sunset Dr. Its $30 million bid is twice as much as was offered in February by Lynx Development Group, but includes more property in return.