How easy can it be?

I have questions regarding the ebay affiliate program.

This is just an idea I have:

Sell $100 amazon gift cards on ebay for $85. Good for the customers who save 15 bucks but also good for me:

I would profit this by promoting my listing on my own website with affiliate links (ebay pays 50% commission).
Profits would be, per sale:
Revenue: 85 - 10(selling fees): $75.
Commission: 85 x 50%: $42.50
Total Revenue: $117.5

Profit: $117.5 - $100 (gift card bought on amazon): $17.5

Why does it look so easy? Am I missing out on something here? full disclosure I'm a complete newb.

Much obliged!