As a seller, do not accept a giftcard code on the chat. Instead, the seller should provide an email in chat. The buyer should buy a new giftcard explicitly for the trade and have it sent to the email address of the seller. This way the seller knows he is the only one to have access to the redemeable code. This apprach also generates verifiable evidence that the giftcard was bought for the RoboSats trade in case of dispute.