CASHlib is a prepaid voucher designed around anonymity: no bank account, no card details, just a 16-digit code you buy with cash and use to pay on supported sites. It's popular for exactly that reason, but it also means there's no official way to redeem it for anything other than what it was designed for.
Why people convert it
If the site you meant to use doesn't accept CASHlib, or you just ended up with a code you don't need, the value doesn't disappear, it just needs a buyer. That's the same situation as any prepaid voucher: someone else takes the code and pays you a percentage of its face value.
Selling it through Giftnovo
Open the CASHlib offer, enter the face value, and start a trade. That opens a chat where you send the code, an admin confirms it, and the payout goes to your wallet right after.
What happens after you send the code
An admin checks the code is real and unused before releasing anything, then the payout lands in your wallet. You can hold it there, withdraw it, or put it toward something in the gift card store.
Staying safe
Because CASHlib is marketed around privacy, it also attracts people trying to buy codes off unverified sellers in chats with zero recourse if things go wrong. If a code is confirmed before payment, and everything happens inside one chat thread, you have a record of the whole exchange if something needs to be disputed later.