Is it still possible for me to sell a gift card(American Express) that still has money on it
for TC?

No, cause you can't recieve the card after the tc is sent.

It would cause a scamming problem.

Ultimately, it depends. I've seen people do it before and there weren't any issues with it. But if I were you, I'd double check back with Gum on this. He would know for certain what the rule is about it.

I am not positive about american express but I sold a couple a while back.(GameStop and iTunes)
But checking with gum would be helpful.

Just gonna repeat what I've said in the pm for everyone to see.

NO!

There is no way for us to verify if you have been scammed if you are a part of this trade. This type of trading is not supported in any way.