I used to buy Apple gift cards in supermarkets etc. Then on ebay i found lots and lots of sellers with great offers, eg a $50 for only $30 or $35.
I mean just a saving like that is obviously a fair few games.
Still unsure how they do it, one seller i spoke to said their overheads are low as they just email you the discount code, so they dont have to bother with the actual card itself.
I never buy Apple gift cards now, always get them online from cheap sites. I've bought about 25 in the last 2 years and havent had one problem with the codes