Since I myself sell gift cards, I wouldn't agree with banning the sale of git cards on the forum. However, there are some obvious users who trade illegal gift cards as well as keys that are bought from stolen credit cards and they sell it at 20-25% face value. I would like that it should stop but still there is no way to prove they are illegit unless the websites are contacted and informed about the same. Also, there are many members who buy these illegally obtained gift cards. Don't get what they get out of it.
I don't agree that it will hurt bitcoins as mainly CC's are used for the purchase of these gift cards and not bitcoins. Don't get what's hurting bitcoins in all this.
I think the discount is a strong way of detecting these carded gift cards. When you see GiftCards at 75 percent... there is a reason on most there are very few people are normally willing to let go for 3/4 of value. Another good way is asking to see receipt of paying with cash... but arguably this could be defeated.
Sadly I think this is a moot point as some are obviously ok with buying carded CG... which is honestly sad. It should catch up with them eventually companies do track addresses of where carded items were sent to.