A couple of thoughts on buying low and selling high. Which I understand you dismissed. I rarely see someone sell low. Instead they either hold because they continue to believe or they outright drop the player. Every year I pick up good players that are dropped. This past year in baseball both Chris Archer and Freddie Freeman come to mind. Ryan Getzlaf and Corey Schneider last winter. Most lack the patience to sit out the sell low.
Sell high, I have seen and done but an interesting twist is the buy high. The other owner is desperate to sell high on a player that you think might sustain his pace and he'll take almost anything in return. Again in baseball, a few years back in his big year I got Chris Davis for Rafael Betancourt (a closer at the time) and recent callup Tony Cingrani.