Escrow.
Funds will be on the escrow while the gift cards will be sent first to the buyer. The buyer will try the gift cards and if it's working then the escrow can release the funds to the owner of those gift cards. CMIIW.
There could be some other methods that can be worked on with the help of an escrow.
You are just so right, I have tried it on paxful before and it went well, on paxful and other exchmges that orobide esrow service, you will see a reputation or trust score, I sell or buy from people with positive reputation with no negattive one, and paxful itself will escrow the fund. Also on some sites like this forum, it is a thing that can be done successfully with the use of escrow as well.
@OP said that we shouldn't mention paxful. I don't know the reason, maybe he has a bad experience with that platform. I haven't used that platform but if there's another p2p transactions platform for this kind of dealing, bitcointalk is the best choice.
A lot of reputable escrow are willing to help to make sales happen from strangers to strangers. But of course, there's a relative fee for their service but it isn't that much to make the the deal safe.