With the rise of online credit card theft, one of the ways fraudsters can turn their theft into cash with limited risk to thmselves is to buy 'virtual' vouchers (sent as a code) with the stolen card details and then sell them on in places like HUKD.
My experience: I agreed to buy a voucher but wanted to hold payment until I verified that the voucher worked. The seller reluctantly agreed based on my feedback. I used the voucher and it appeared to work fine, but the retailer marked the item I wanted as out of stock, so I asked my seller to wait until I could verify they would get it back in stock. The seller tried to demand payment, but I held firm. When I phoned the retailer to check if/when stock would arrive, I was asked questions about where I got the voucher. Long story short, the day before one of their customers reported the voucher purchased without their knowledge on their credit card.
If you can get the seller's full details and can verify them then you should be ok, but be especially careful of sellers who give only details such as an ebay account and refuse to give an address and landline telephone number.