Not that I expect the other person to fess up to redeeming them, but have you contacted them regarding the problem? If you get no response, I believe the situation calls for negative feedback as you were essentially scammed.
I guess I need some advice on a trade, I traded Stardew Valley and Octodad for Total War Warhammer, but the gift link I was sent had already been redeemed when I clicked on it. The other person said I should just keep my games and that they hadn't used them yet, but both of the gift links I sent them now say they have been redeemed already. Not sure what to do now.
If you get a response saying they didn't redeem the codes or don't know what's wrong, neutral feedback would be appropriate since you were still wronged but not explicitly so.
This sort of situation is why it's recommended that the user with the highest feedback verifies that their side of the transaction checks out before carrying on with full completion.