From my recollection when you open a dispute you are able to send video/photographic evidence to assist Gameflip with making a decision. I would suggest sending them a copy of the PSN Gift Card/Code and a receipt if you have it.
In this particular thread I would suggest adding the invite code from your profile to assist.
Hopefully, you used the auto-trade function, in which case the buyer has already transferred the money and Gameflip will likely find in your favour and the money will be sent to you (just be patient). If it was a manual transaction then you will likely find it extraordinarily difficult to get your money. The scammer will likely be suspended.
You could also contact Sony if you don’t get your money and report that the code has been lost/stolen. I would be careful how you phrase any correspondence to them as they may have terms and conditions prohibiting the resale of PSN Codes. They may be able to take some action against the fraudster/scammer from their side.