Digital asset backed by amazon gift cards

I want to create a digital asset backed by Amazon Gift Cards.

Amazon Gift Card are great because they are redeemable toward millions of items at Amazon.com, have no fees, and NEVER expire.

Is it technically feasible a digital asset with the following specifications:
- I buy all Amazon vouchers and encrypt them somewhere in the blockchain
- I create the initial coins which will be backed by these vouchers
- Coins property can be transferred between users
- A function "convert" will allow a user to destroy coins and get amazon voucher in exchange

For this last point, this is my problem, is there any existing technology/protocol which would allow a user to redeem his coins but without me acting as a bot/oracle.... ? It would be great that users can still redeem their vouchers, should deseappear