Nothing much can we do to block people trying to commit the fraud - blocking single IP is a waste of effort, I can bypass such a block in 5 minutes, so could fraudsters.
The only possible thing is to make forum buy/sell forum invite-only, but that defeats the growth
Some buyer's due diligence and common sense will do miracles:
1. Be careful with new sellers, request references and/or receipts
2. Be careful with super low prices, remember - fraudster works off near zero cost basis, so he will undercut any legit sellers
3. Report serious problems immediately to community and mods
4. Remember even reputable sellers might have mistyped PIN/card # or not properly activated card, if it's a single or rare occurrence and addressed immediately we're still good.