It's not only you I have been trying to purchase a pre-owned Samsung Galaxy S7 Edge and have had the same issues. I have contacted my bank a card provider and they have confirmed that there are NO issues at their end! Having emailed an GG agent they explained "Terry here from the finance team. I have had a look into this for you and it looks like an issue at our end has been stopping the payments. You'll be pleased to hear I have now managed to put a fix in place that will allow you to use your card going forward. It can however take about 20 minutes for the fix to kick in.
Please be sure to use the card ending XXXX and you should have no problems. These precautions are in place to combat any potential fraud issues which I'm sure you can understand but I'm sorry for any inconvenience this may have caused."
I have then tried to comp-lete the purchase as instucted but recieved this message, "Unfortunately, due to an error in our system we were not able to process your order. Please try again in a few minutes by visiting our phones range page."
AT LEAST THE BANK IS NOT GETTING THE BLAME THIS TIME!