Gift card schemes are becoming very common these days. You will receive a mail stating that you just won $25 GC or so and to verify that you will have to give personal details to the sender. Some scammers are ready to give you $25 GC for just $10 and so on. The best way to deal with these emails is to delete it right away and also mark them as spam mails. You should never click on the links given on these mails. But there are genuine stores too who give away valid gift cards to their loyal customers. It is really difficult to filter the right ones. If you don't know the sender try to verify it over phone or through other ways before going ahead.
Last week, I've received invitations from couple of places to take short surveys. On completion, they'll offer 20$ gift cards. It looked fishy as I'm not registered with any of these sites. However, I did not take the surveys though the amounts were lucrative. So far, I was not aware of such GC scams. Now understand that the invitations could be traps of similar kind.
By experience, I have been a victim of such scam. The sender stated clearly that the gift card came from a legitimate company. I didn't have any second thought. I gave right my personal details until I reached to the point of giving my credit card number. I stopped for I suspect something was messy. It was good that I didn't receive any follow up from them. So I said to myself to be extra careful and don't take any action right away without checking or verifying the source.
If you haven't signed up for a loyalty scheme for a shop, be very suspicious about someone emailing you offering giftcards. If you have signed up, most of the stores will just send you a giftcard or voucher in the email that is linked to your loyalty card. The other way to tell is to mouseover, but don't click on, the links. Depending on your browser it will bring the URL where the link goes to up in the status bar, and usually if this is a scam at least one of the links will not be to the store's homepage. And if someone says they want to give you money, never give them yours or your card details!
I have always just followed the rule of thumb where if there is more than two or three steps than it is probably a scam or just not worth it. These seem to fall into that category, at least most of the time.
Not only are the gift cards mentioned in this thread scams, but also the ones bought on eBay. For instance, even though the fast food cards work at company-owned stores, I feel the origin of the cards is shady. Therefore, I don't want to continue to use them for too long - cause I think it will arouse suspicion and eventually, even the company owned stores will probably try to crack down on this stuff.
You also have to be careful with sites like Amazon although it is not a scam. I once chose an Amazon gift card as a reward from a chat site. Unfortunately I realised it was for Amazon.com, the American site and I am from the UK so the voucher was useless to me. They said that once it was issued they could do nothing about it.
There really do seem to be quite a bit of these that have really popped up over the past couple of years. That is good when they display the proof of payments though, that makes me feel a little better.