As an addendum to this, if you are buying a gift card at less than cost price, then you are almost certainly buying a gift card which has been stolen, or bought with stolen ("carded") credit card details.
Depends on the discount. 20% or more, unless it's a $5 card or something close, it's very likely it's been fraudulently obtained. The higher the value of the transaction the lower should the discount be. Most discounts via purse.io are ~5% now if you're a new user.