HI Nick,
yes the mods have seen your report and as Bill has pointed out, makes it hard for any comment when we do not have much in the way of details from this post.
1. Best thing you can try to do is to make contact with the seller. Try and come to some sort of refund/return arrangement.
2. If you have any other details try other contact means (facebook, email), try others he has recently traded with.
3. Leave appropriate feedback. if the seller did not protect the card in an appropriate manner.
4. if you paid via paypal you could also investigate that route if the seller doesn't come to the party.
and if it is the one that I believe it is, the person has not made a post since agreeing to your trade thread.
whilst it has shown they have been online, they may not have been (it could have been their homepage which someone else has accessed) or not in a position to reply, I often access the site at work on my phone but cannot easily reply.