I believe that you are not a scammer, but since Apple doesn't know you bought all of these cards yourself, you exactly fit the profile of someone who scams people and has them send him Apple gift cards over a long period of time, and then he redeems them all at once, thousands worth. Your behavior should have tripped automated systems, and any humans looking at it should have been very suspicious as well.

Like I said, I believe you're not a scammer, although Apple's folks have more experience in dealing with scammers than I do, and they clearly think you are. My belief is based mostly on the fact that you say you have receipts.

The real shame here is that Apple provides no clear appeals process for you to show your receipts and prove you bought these cards yourself.