With older players it often depends on the league set up. I would say more players you keep, less valuable old guys are. In Dynasty league Stamkos wont fetch more than A- prospect or 1st rounder outside of lottery pick. ITs not right but its reality and in that case you are better off to just keep the player.
If you in limited keep 5 league lets say, Stammer most likely is not a keeper but guy you acquire for him is not a keeper either so people are more likely to invest.