I buy moondollars when there is a reasonable fall of the price. If the fall of the price is near the common ups and downs of the price, I will not buy any. Other than that, I do not buy moondollars regularly because I earn it. 
I sometimes do that, if I still have spare money, I grab the opportunity when the price drops more than normal, say about $20 drop, but now, I think it is good to buy when the price drops atleast $5 and same as selling..