Do note however, there isn't a way to tell if a vacant plot is available for purchase other than going to the placard and trying. At best, you can take a rough guess how soon it will be available by considering the average time until the lock is lifted and how many times the plot has devalued. You can check this by looking at the original price and the current price. Each devalue lowers the price by 0.42% and the plot is devalued every six hours. A plot that has devalued six times, has been up for 36h and should be unlocking soon.