I've recently been upgrading and repurposing some older hardware to rebuild some internal infrastructure, so it now all sits on top of Proxmox running on a couple of NUCs. This has left me with a few components which, given the price of RAM and storage, are actually well worth the hassle of selling on eBay.
Selling old memory is trivial enough, but selling old storage devices requires a bit more care. Most people these days know that simply deleting files is completely insufficient if you want to prevent data recovery, as is simply creating a fresh filesystem on the drive.
Many modern NVMe drives make this easier. Those that support cryptographic erase can securely wipe data simply by replacing the encryption key used to protect it, making the old data effectively unrecoverable.
Not all drives support this capability, however. If your storage is older, lower-end, or simply lacks cryptographic erase support, then you need another approach. This is where secure wiping tools come in handy. The one I use is called RedKey USB.
This is a bootable USB stick that will wipe any storage attached to the system you boot it on. It can also identify the type of storage present and determine which devices support secure erase operations and which require a more thorough overwrite process.
TL;DR: Don't sell storage to a stranger unless it has been properly and securely wiped first.