What is a key trade piece? It's complicated.
For a contending team, it might be a high-quality backup who could start elsewhere and fetch a player who could fill another hole.
A key trade piece for a rebuilding team might be an aging veteran who would be more highly valued by a contender and could fetch an extra draft pick or two.
For a team flush with cap space, it could be cheap player who could be swapped for a better, more expensive one.
And a key trade piece for a team low on cap space could be an expensive player the franchise could swap for someone cheaper and less established with more upside or an extra draft pick or two.