The owner of Denver's Six Flags Elitch Gardens amusement park is seeking $170 million for the 75-acre site, according to a real estate developer who has seen the offering. Six Flags Inc., which operates 30 parks in North America, is saddled with more than $2 billion in debt and is considering selling Elitch Gardens and five other parks. It plans to send out marketing materials on the properties to interested buyers this week.