The Steam Deck is still quite popular, and so a used Steam Deck 512GB SSD in good condition can be sold for $400 to $450 on average. Meanwhile, a 256GB Steam Deck in good condition tends to go for $350 to $400, and the 64GB Steam Deck is typically worth $300 to $350 if in good condition.
However, if there is noticeable wear on your gaming handheld or there are issues with it then its value drops significantly. Additionally, if you do sell to a used video game store, they will give you less than these amounts to make a profit when they sell the device to someone else.