First of all Gameflip don’t cheat, second if code doesn’t work or it’s been already used there is a opportunity as buyer to complain.
After purchase you have exactly 3 Days after delivery to inspect the item and rate the seller. If you dont rate him at 3 days, Gameflip will issue as Auto-rate and transaction will be completed.
As buyer, you can open a dispute against the seller, sending the proper evidence to support, and waiting for their response. Dispute will stop the time and make your transaction long enough for stuff to review it. If your evidence is promising and valid, transaction will be refunded in your favor. If your evidence is not strong enough and it might be suspicious they will ask for some more info and probably complete the transaction in seller’s favor.
On pictures like this, try to avoid showing the code. edit the picture in paint and then post it.
Few link for you to read for next time.
First is about buying LINK#1 ( scroll down for section of COMPLETING TRANSACTION )
Second is about disputes LINK#2(How to open them, and mostly how do they work)
Third link is about contacting the support LINK#3(Link for new submit/request)
Thats all for now, be careful next time. Inspect every item before rating, from any seller, anyone can make a mistake, post a wrong code, miss one letter/number of code and make it unusable etc etc. Just make sure you get what you have paid for.