Preservation of value when selling vouchers?

I was just wondering something strange:
let's say you buy a voucher or for Amazon or something.
You can also sell it back to private or to corresponding sites, but at a discount.

Now there are various providers, netflix, google play, amazon, etc. For which you can buy credit with a voucher.

Now I would have been interested in who the discount is the lowest in percentage terms,
So who can I sell their vouchers to and get most of the money back?

The ECB's universal vouchers have the lowest loss of value.

What do you mean by that?

https://de.m.wikipedia.org/...obanknoten

Oh: -D