Raise.com can save you MASSIVE amounts of money when doing online or retail arbitrage. They offer discounted gift cards with a large % of savings, and their supply is usually pretty high. You can also sell gift cards you don’t use to others.
Basically, if you know you are going to spend a certain amount at a retail store, you can buy a gift card for that store at a % off of the gift card value. If you will spend $100, you can buy a $100 gift card for $85-95 instead.