This really depends on how you want to handle things. Fewer used video game stores are willing to buy a Steam Deck from you, and they will usually give you far less than if you sold the Steam Deck to someone else yourself. However, selling your Steam Deck on eBay, Facebook Marketplace, or any other consumer-to-consumer site could take more effort and might not sell right away.
If you're interested in making more money off of your Steam Deck sale, then it's best to sell to another user yourself rather than selling to a store.