This just in from Tina Mammoser.
We're getting more "gift" and "prepaid" credit cards here now so it's relevant. If you do take a giftcard Visa or similar at an art or craft fair or other event be sure to process it right away. These are basically just pre-loaded cards. The problem can be if the person buying doesn't pay attention to their balance right and continues purchasing on it later in the day, before you process payment – then your payment can fail later. The preloaded and gift cards just take money off the balance as each transaction is processed.
One way of spotting these is that they don't have a name on them since they're generic and preloaded. (I use them a bit in the US, friends and family give my money that way rather than cash.)
It's not that the buyers are at all dishonest! Just that misjudgments in spending can happen.