Not everytime there are USD420 available. You cannot use a coupon beyond 50% of the miner price. e.g. in order to fully utilize the USD420 coupon, the miner should be above USD840. What this means is that if you think you can purchase a S9 (USD499), you would not be able to use the coupon. You cannot expect to pay USD79 for the S9.
I am selling genuine coupons USD55 which I have had successful transactions. A lot of other high value coupons, are something you need to double check on the authenticity.
https://bitcointalk.org/index.php?topic=4938455.0
