My dad says that Marriott was going to sell the park for its land value in 1984; however the city saved the park when they bought it and contracted KECO to run it for them.
As for the Gurnee park, I am pretty sure Six Flags was not who Marriott sold it too. I'm getting old but I think it was Bally Corporation that might have been the one that purchased that park???
As for Chicago, that was a direct deal between Marriott and Bally, which owned the Six Flags chain at the time.