I've sold CDs to "SecondSpin.com".
You can type the SKU number into the site, and it'll tell you instantly what they'll pay you for it.
Last time I sold them discs, they paid for "media mail" shipping, and it didn't cost me a dime. I could use the money towards buying music from their site, or cold hard cash.
Or...take it to your local used music place and let them give you an offer.
You can also start an Ebay auction for a "lot" of heavy metal CDs.
A "lot" as in a 'group', as opposed to a 'bunch', if you catch my drift.
Back in the day, I bought CDs from Ebay, and used the search term "lot", as it gave me larger quantities of discs.
Best of luck.

Bob