Karlsson has huge value in 6D league formats.
The only league where I'd sell high on Karlsson right now is multi-cat with Hits/BkS/PIM.
In points-heavy leagues, I think Karlsson should be kept.
His value is farther above waiver-wire defensemen than other ELITES relative to their waiver-wire positions.
In other words, you have to get two fairly elite players back to trade EK away.